Chapter 3.6, Problem 14E

Let A be a $4\times 4$ matrix with reduced row echelonform given by

$U=\left(\begin{array}{cccc}1& 0& 2& 1\\ 0& 1& 1& 4\\ 0& 0& 0& 0\\ 0& 0& 0& 0\end{array}\right)$
If ${\text{a}}_{\text{1}}=\left(\begin{array}{l}-3\\ 5\\ 2\\ 1\end{array}\right)$ and ${\text{a}}_2=\left(\begin{array}{l}4\\ -3\\ 7\\ -1\end{array}\right)$
find ${\text{a}}_3$ and ${\text{a}}_4$ .