申論題

14.  Let matrix $A \in \mathbb{R}^{3 \times 4}$ have its $A_{ref} = \begin{bmatrix} 1 & 1 & 2 & 0 \\ 0 & 1 & -2 & -1 \\ 0 & 0 & 0 & 0 \end{bmatrix}$. 

(c) If $\mathbf{x} = \begin{bmatrix} 3 \\ 2 \\ -1 \\ 1 \end{bmatrix}$ solves $A\mathbf{x} = \mathbf{b}$, find the solution to $A\mathbf{x} = \mathbf{b}$ that has the smallest 2-norm.