Show that $\displaystyle \frac{(-1+(a)^2)}{(2-y)}=\frac{17}{(2+y)^3}$ where

$\displaystyle a=\frac{(1-x)}{(2-y)}$ and also $\displaystyle x^2 - y^2 - 2x +4y -20 =0$

Can anyone do? lol

p.s. This is the end simplification of another problem which I just don't know how to do.

Looking in particular to learn useful methods to do this efficiently for future reference.

Many thanks in advance!

Edit: Still need help...