4. (10 points)
Determine if the subsets $W$ of $\mathbb{R}^3$ defined below are subspaces of $\mathbb{R}^3$.
If so, give a proof; if not, explain why.
(a) $W = \left\{ \begin{bmatrix} t+s \\ 5t \\ t+3s \end{bmatrix} \in \mathbb{R}^3 \right\}$.
(b) $W = \left\{ \begin{bmatrix} a \\ b \\ c \end{bmatrix} \in \mathbb{R}^3 : b+c \ge 0 \right\}$.