What is a 3×4 coefficient matrix  such that [|⃗ ] has a solution for every 3×1 vector ⃗. I know that A needs a pivot in every row and that every column vector b (with m entries) is a linear combination of the columns of A. However, I am stuck on giving an example of a matrix that would have a solution for every b vector.

$$A=\begin{bmatrix}1&0&0&0\\0&1&0&0\\0&0&1&0\end{bmatrix}$$
