public class SignUpActivity extends AppCompatActivity {

FirebaseAuth mAuth;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_sign_up);
    mAuth=FirebaseAuth.getInstance();
}

public void fgh(View view) {
    mAuth.createUserWithEmailAndPassword("example@email.com","password")
            .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
                @Override
                public void onComplete(@NonNull Task<AuthResult> task) {
                    if(task.isSuccessful())
                    {
                        Toast.makeText(SignUpActivity.this, "Successfully created your account", Toast.LENGTH_LONG).show();
                        Intent intent = new Intent(this,SuccessActivity.class);  //this is my error
                        startActivity(intent);  // how to start activity here
                    }
                    else{
                        Toast.makeText(SignUpActivity.this, "ERROR has occurred", Toast.LENGTH_LONG).show();
                    }
                }
            });
}
}

Intent intent = new Intent(SignUpActivity.this,SuccessActivity.class);

Intent intent = new Intent(getApplicationContext(), SuccessActivity.class);

private Context context;

context = this;