I am a bit rusty on this but I think I know how to do it:
Try to find rcos(theta) or rsin(theta) in the equation and replace it with x or y and replace r2 with x2+y2. Often you will have to multiply both sides by something so that you can find these expressions.
In this case r sin q = -2
r sin(q) = -2
y = -2
Am I right anyone?