$\displaystyle x^2 + 4y^2 = 9$

Show that $\displaystyle \frac{d^2y}{dx^2} = \frac{45}{2(x+y)^3} $.

I was wondering if I could get some help with it. I get the derivate to be $\displaystyle -\frac{x}{4y} $ (which is correct) but when I try to differentiate this again I get a completely different answer to that in the question. Thanks, any help would be much appreciated.