$\displaystyle -3x^2+1xy-1y^3=-15$

I have to find the slope of the tangent line at the point of (-2,1)

Now from my work, the derivative of this function is

$\displaystyle dy/dx = (-6x+y)/(3+3y^2)$

Therefore the slope = 13/6

But the answer ain't right, why?

If anyone can help before I head off to the exam tomorrow morning at 9:30am I will really really really really appreicate it!!!

THANK YOU VERY MUCH!!!