Given $ A = \begin{bmatrix} 1 & -1 & 1 \\ 1 & -2 & -2 \\ 2 & 1 & 3 \end{bmatrix} $ and $ B = \begin{bmatrix} -4 & 4 & 4 \\ -7 & 1 & 3 \\ 5 & -3 & -1 \end{bmatrix} $. Find `AB`. Hence from this result, solve the following set of equations. `x-y+z=4`, `x-2y-2z=9`, `2x+y+3z=1`