You can let webwork do much of the calculation for you if you want (e.g. enter 45-(56/76)(-3) instead of evaluating it and then entering it). You can also use the preview feature in order to make sure that you have used the correct syntax in entering the answer.
(Note: since the determinant is unchanged by row reduction it will be easier to calculate the determinant of the row reduced matrix.)
You can earn partial credit on this problem.