GMAT Data Sufficiency (DS) | If p and q are distinct lines in the xy coordinate system

If p and q are distinct lines in the xy coordinate system such that the equation for p is y= ax + b and the equation for q is y= cx + d, is ac = a²?

(1) d = b + 2
(2) For each point (x, y) on p, there is a corresponding point (x, y + k) on q for some constant x.

I read this question in Princeton Review. In answer part, they show this:
ac = a² so a = c. But if a = 2 and c = -2, for example? I can't get it.