3.11: Cosine graph: find a, b and c

The diagram shows the curve $y = a\,\mathrm{cos}\,bx + c$ for $-180^{\circ} \leqslant x \leqslant 180^{\circ}$. It is given that $a$, $b$ and $c$ are integers. Find the values of $a$, $b$ and $c$.