Implicit Differentiation problem

[123rock]

I can't get the second derivative for this:

x^3 - 3xy + y^3=1

The book says that the answer is -4xy/(y^2-x)^3

but this is what I get:

http://www.geocities.com/uganda_trader/implicit.htm

Thanks

[OfficeShredder]

Differentiate implicitly, but don't solve for y'

Instead, try just differentiating again. Then solve for y'' in terms of x, y, and y', and plug your y' solution into your y'' solution (I didn't actually work this out, but it probably looks much nicer, with less room for error)