Error starting Activity

Process: com.example.imatematik02, PID: 2358 android.content.ActivityNotFoundException: Unable to find explicit activity class {com.example.imatematik02/com.example.imatematik02.GameActivity}; have you declared this activity in your AndroidManifest.xml? at android.app.Instrumentation.checkStartActivityResult(Instrumentation.java:1794) at android.app.Instrumentation.execStartActivity(Instrumentation.java:1512) at android.app.Activity.startActivityForResult(Activity.java:3917) at android.support.v4.app.BaseFragmentActivityJB.startActivityForResult(BaseFragmentActivityJB.java:48) at android.support.v4.app.FragmentActivity.startActivityForResult(FragmentActivity.java:75) at android.app.Activity.startActivityForResult(Activity.java:3877) at android.support.v4.app.FragmentActivity.startActivityForResult(FragmentActivity.java:856) at android.app.Activity.startActivity(Activity.java:4200) at android.app.Activity.startActivity(Activity.java:4168) at com.example.imatematik02.MenuActivity.startPlay(MenuActivity.java:41) at com.example.imatematik02.MenuActivity.access$000(MenuActivity.java:13) at com.example.imatematik02.MenuActivity$1.onClick(MenuActivity.java:29) at com.android.internal.app.AlertController$AlertParams$3.onItemClick(AlertController.java:1108) at android.widget.AdapterView.performItemClick(AdapterView.java:310) at android.widget.AbsListView.performItemClick(AbsListView.java:1145) at android.widget.AbsListView$PerformClick.run(AbsListView.java:3042) at android.widget.AbsListView.onTouchUp(AbsListView.java:3891) at android.widget.AbsListView.onTouchEvent(AbsListView.java:3656) at android.view.View.dispatchTouchEvent(View.java:9294) at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2547) at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2240) at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553) at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2254) at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553) at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2254) at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553) at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2254) at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553) at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2254) at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553) at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2254) at com.android.internal.policy.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:2403) at com.android.internal.policy.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1737) at android.app.Dialog.dispatchTouchEvent(Dialog.java:787) at com.android.internal.policy.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:2364) at android.view.View.dispatchPointerEvent(View.java:9514) at android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent(ViewRootImpl.java:4230) at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:4096) at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3642) at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3695) at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3661) at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:3787) at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3669) at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:3844) at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3642) at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3695) at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3661) at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3669) at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3642) at android.view.ViewRootImpl.deliverInputEvent(ViewRootImpl.java:5922)