welcome to the Ranch and enjoy the stay!
It is me a little unclear as to what the purpose is of the classes Equation and Reponse, so perhaps you can elaborate a bit.
In the class Reponse, you make an instance of Equation, that determines x, y , z and an operator op such that x op y = z. Okay, you then display that equation,. Next, you determine a value for a, or b, or c, or d.
And then? What is supposed to happen next? Must a user make some guess? And if so, what guess?