Solution (source code)

= Solution
{c}

By definition, the <image of a linear map> $T_A$ is
$$
\operatorname{im}T_A=\{A\mathbf x:\mathbf x\in\mathbb R^m\}.
$$
Consequently $\mathbf b\in\operatorname{im}T_A$ means exactly that some $\mathbf x\in\mathbb R^m$ satisfies $A\mathbf x=\mathbf b$. This is precisely the existence of a solution of the <linear system>.