Show that if $W$ and $Z$ are subspaces of $V$, then (a) their intersection $W \cap Z$ is a subspace of $V,(b)$ their sum $W+Z=\{\mathbf{w}+\mathbf{z} \mid \mathbf{w} \in W, \mathbf{z} \in Z\}$ is also a subspace, but (c) their union $W \cup Z$ is not a subspace of $V$, unless $W \subset Z$ or $Z \subset W$.