A 6×46 \times 46×4 matrix AAA has rank 3. For the system Ax=bA\mathbf{x} = \mathbf{b}Ax=b, when does a solution exist?
For all vectors b\mathbf{b}b
Only when b\mathbf{b}b lies in the column space of AAA
Never, since rank is less than the number of rows
Always, with exactly one solution