(a) Show that if $\mathbf{v}$ is a linear combination of $\mathbf{v}_1, \ldots, \mathbf{v}_m$, and each $\mathbf{v}_j$ is a linear combination of $\mathbf{w}_1, \ldots, \mathbf{w}_n$, then $\mathbf{v}$ is a linear combination of $\mathbf{w}_1, \ldots, \mathbf{w}_n$.
(b) Suppose $\mathbf{v}_1, \ldots, \mathbf{v}_m$ span $V$. Let $\mathbf{w}_1, \ldots, \mathbf{w}_m \in V$ be any other elements. Suppose that each $\mathbf{v}_i$ can be written as a linear combination of $\mathbf{w}_1, \ldots, \mathbf{w}_m$. Prove that $\mathbf{w}_1, \ldots, \mathbf{w}_m$ also $\operatorname{span} V$.