If m= f(y) and y= g(x), then so the answer to your question is "No". That product will be 0 if and only if at least one of or is 0- in other words if m is NOT a function of y or y is NOT a function of x. Your real problem is that you havetwo"unkowns", m as a funciton of y and y as a function of x, but only one equation.