Suppose the RREF of an augmented matrix $[A \mid \mathbf{b}]$ is:$\begin{bmatrix} 1 & 0 & 2 & 1 & | & 5 \\ 0 & 1 & -1 & 3 & | & 2 \\ 0 & 0 & 0 & 0 & | & 0 \end{bmatrix}$\nExpress the general solution in parametric form, where $x_3 = s$ and $x_4 = t$.\nWhich expression for $x_1$ is correct?