I'm having trouble with this problem.

The tangent to the curve (y^2)-xy+9=0 is vertical when

(a) y= 0

(b) y= +/- radical 3

(c) y= 1/2

(d) y= +/- 3

(e) none of these

I took the derivative of the equation and solved for x' and I got

x'= (x-2y)/(-y)

and then I set it to 0 but I get

x-2y=0

and I don't know what to do from there.