I have the following function
$f(x,y) = \log_2 (aby - ab^2y + acy -2abcy +ab^2cy - ab^2 + ab^2x + b - bx - abc + ab^2c + abcx - ab^2cx)$
I wanted to find $(x,y)$ that maximizes this function. However, when I compute the first derivative, with respect to x and set it to 0, I get the following
$$\frac{\partial f}{\partial x} = \frac{ab^2 - b + abc - ab^2c}{aby - ab^2y + acy -2abcy +ab^2cy - ab^2 + ab^2x + b - bx - abc + ab^2c + abcx - ab^2cx} = 0$$
As you can see there's no $x$ nor $y$ in the numerator. How am I supposed to get the critical point in this case?