Solution
= Solution
For $v\in V_\lambda$, use commutation to obtain
$$
T(Sv)=S(Tv)=S(\lambda v)=\lambda Sv.
$$
Thus $Sv\in V_\lambda$, proving that \b[every $T$-<eigenspace> is an $S$-<invariant subspace>]. This does not assert that an arbitrary <eigenvector> of $T$ is already an <eigenvector> of $S$.