For $\displaystyle \pi<a<\frac{3\pi}{2}$ with $\displaystyle cos(a) = -sin(b)$ where $\displaystyle 0<b<\frac{\pi}{2}$, find $\displaystyle a$ in terms of $\displaystyle b$.

I am terrible at maths so please show any simple easy to understand working out so that I can understand better.

Any help would be appreciated!