Let $F$ and $G$ be two supplementary subspaces of $E$ and $s$ the symmetry with respect to $F$ parallel to $G$.\\
a) Show that $F = F_s$ and $G = G_s$.\\
b) Show that $s \circ s = \operatorname{Id}_E$. Deduce that $s$ is an automorphism of $E$.\\
c) Determine the eigenvalues and eigenspaces of $s$. We will discuss according to the subspaces $F$ and $G$.