Let $s$ be an endomorphism of $E$ such that $s \circ s = \operatorname{Id}_E$. We set $F = \operatorname{Ker}(s - \operatorname{Id}_E)$ and $G = \operatorname{Ker}(s + \operatorname{Id}_E)$.\\
a) Show that $F$ and $G$ are two supplementary subspaces of $E$.\\
b) Deduce that $s$ is a symmetry and specify its elements.