how to delete a firebase record from android studio?

.I've done an app where you insert data from android studio to firebase with 3 EditText so

and what I need is to delete data but I do not know how to do it, the truth is that I am still new and I would need help ... the deletion could be just by consulting only the id or all the data

this is my MainActivity class

public class MainActivity extends AppCompatActivity {
private EditText Alias;
private EditText Nombre;
private EditText ID;

public static int javo=0;
public static ArrayList<Ccontacto> ccontactos = new ArrayList<Ccontacto>();

private Button btnagregarfb,btnlistafb,btnEliminar;
private FirebaseDatabase database;
private DatabaseReference databaseReference;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    database = FirebaseDatabase.getInstance();
    ID =(EditText) findViewById(R.id.txtId);
    Nombre=(EditText) findViewById(R.id.txtNombre);
    Alias =(EditText) findViewById(R.id.txtAlias);

    databaseReference = database.getReference("Contactos");
    databaseReference = database.getReference("Contactos");
    database.getReference().getRoot().addChildEventListener(new ChildEventListener() {
  @Override
  public void onChildAdded(DataSnapshot dataSnapshot, String s) {
      Toast.makeText(getApplicationContext(),"Contacto agregado", Toast.LENGTH_LONG).show();
  }
  @Override
  public void onChildChanged(DataSnapshot dataSnapshot, String s) {
  }
  @Override
  public void onChildRemoved(DataSnapshot dataSnapshot) {
  }
  @Override
  public void onChildMoved(DataSnapshot dataSnapshot, String s) {
  }
  @Override
  public void onCancelled(DatabaseError databaseError) {
  }});
    btnagregarfb = (Button) findViewById(R.id.btnagregarfb);
    btnagregarfb.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            javo++;
            int idcontacto = javo;
            String nombre = null;
            String alias=null;
            nombre = Nombre .getText().toString();
            alias = Alias.getText().toString();
            Ccontacto c = new Ccontacto(idcontacto,nombre,alias);
            databaseReference.push().setValue(c);
        }
    });
    btnlistafb = (Button)findViewById(R.id.btnlistafb);
    btnlistafb.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

            Intent intent = new Intent (MainActivity.this,MainLista.class);
            startActivity(intent);
        }
    });
    btnEliminar = (Button) findViewById(R.id.btnEliminar);
    btnEliminar.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
 //Aqui es el evento de click de mi boton eliminar
        }
    });

}}

and this is my constructor class

public class Ccontacto {
int idcontacto = 0;
String nombre = "";
String alias = "";

public Ccontacto() {
}

public Ccontacto(int idcontacto, String nombre, String alias) {
    this.idcontacto = idcontacto;
    this.nombre = nombre;
    this.alias = alias;
}
public int getIdcontacto() {
    return idcontacto;
}

public void setIdcontacto(int idcontacto) {
    this.idcontacto = idcontacto;
}

public String getNombre() {
    return nombre;
}

public void setNombre(String nombre) {
    this.nombre = nombre;
}

public String getAlias() {
    return alias;
}

public void setAlias(String alias) {
    this.alias = alias;
}}