In your program.cs file you would do something like:
Arithmetics arith = new Arithmetics();
arith.Arithmetic();
But in your arithmetic class you have some problems, namely that the functions never really return anything. In order to do it with the parameter in the prototype like you have, you have to declare them with the "out" keyword, like:
public void Compute(out float ans, int number1, int number2)
But the best way to do it is to return the value from the function rather than use it in the prototype, like this:
public float Compute(int number1, int number2)
{
float ans = 0.0f;
return ans;
}
You also want to use more descriptive names instead of "compute", like add/subract, etc. That way you can use the same prototype for each one and not have the type in the parameter list.