Ok, so here goes another problem:
Instructions say to block multiply:
Im stuck at the end where there is clearly something wrong.
Matrix B is: $\begin{bmatrix} 1 & 2 & 0\\ -1 & 1 & 0\\ 0 & 0 & 1\\ 0 & 0 & 3\end{bmatrix}$
..in case its not legible. Sorry for the bad photo :).