Write the system of equations as an augmented matrix
$\displaystyle {\left\lbrace\begin{array}{ccccccc} -{5}{a}&+&{3}{u}&&&=&{350}\\-{2}{a}&+&{u}&&&=&{100}\\{3}{a}&-&{5}{u}&+&{r}&=&{400}\end{array}\right.}$