Given the system of linear equations, \begin{align*} x_1+2x_2 + x_4&= 7\\ x_1+x_2+x_3-x_4&=3\\ 3x_1+x_2+5x_3-7x_4&=1 \end{align*} we have $n=4$ variables and $m=3$ equations. Also, \begin{align*} a_{11}&=1 & a_{12}&=2 & a_{13}&=0 & a_{14}&=1 & b_{1}&=7\\ a_{21}&=1 & a_{22}&=1 & a_{23}&=1 & a_{24}&=-1 & b_{2}&=3\\ a_{31}&=3 & a_{32}&=1 & a_{33}&=5 & a_{34}&=-7 & b_{3}&=1 \end{align*}
Additionally, convince yourself that $x_{1}=-2$, $x_{2}=4$, $x_{3}=2$, $x_{4}=1$ is one solution (Definition SSLE), but it is not the only one! For example, another solution is $x_{1}=-12$, $x_{2}=11$, $x_{3}=1$, $x_{4}=-3$, and there are more to be found. So the solution set contains at least two elements.