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
and then I set it to 0 but I get
and I don't know what to do from there.