a) if x^3 + y^3 -xy^2 =5 , find dy/dx

b) find all points where the tangent line is horizontal or vertical

I was able to do the first part, and I got dy/dx = -3x^2 +y^2/3y^2-2xy.

However, I'm having some problems finding where the tangent line is either horizontal or vertical....... can I get some help.Thanks in advance