@Override
    public void onBackPressed(){
        AlertDialog.Builder dlgAlert  = new AlertDialog.Builder(this);
        dlgAlert.setMessage("Keluar dari latihan soal?");
        dlgAlert.setTitle("Peringatan");
        dlgAlert.setPositiveButton("OK", new DialogInterface.OnClickListener() {
            public void onClick(DialogInterface dialogInterface, int i) {
                Intent home=new Intent(Persamaan.this, MainActivity.class);
                startActivity(home);
                finish();
            }
        });
        dlgAlert.setNegativeButton("Batal", new DialogInterface.OnClickListener() {
            public void onClick(DialogInterface dialogInterface, int i) {
            }
        });
        dlgAlert.setCancelable(true);
        dlgAlert.create().show();
    }