Not too sure where to start with this, any help would be great.
I have calculated the partial derivitives wrt x and y, but think I am missing the formula where I have to sub in.
Thanks
Use "implicit differentiation". If f(x, y)= C (a constant) then $\displaystyle \frac{\partial f}{\partial x}+ \frac{\partial f}{\partial y}\frac{dy}{dx}= 0$. That is, $\displaystyle \frac{dy}{dx}= -\frac{\frac{\partial f}{\partial x}}{\frac{\partial f}{\partial y}}$.