If you know that A lies on the given circle, then you already know the answer to part (a).
If you need the working:
x^2+y^2=\frac{\left(1-a^2\right)^2+(2a)^2}{\left(1+a^2\right)^2}
=\frac{1-2a^2+a^4+4a^2}{\left(1+a^2\right)^2}
=\frac{1+2a^2+a^4}{\left(1+a^2\right)^2}...