You should know that when you can only multiply matrices when the number of columns in the first is equal to the number of rows in the second, and the resulting product matrix will have the "remaining" dimensions as its order. So that means .
Anyway, the idea will be to show that two matrices are equal to the same thing, and are therefore equal to each other. Please note that where I'm using the inverse matrix notation, it means either a right-inverse or a left-inverse, i.e. some matrix which will multiply to give the identity without the original matrix necessarily being square. Working on the first product we have
Working on the second product we have
So it should be clear therefore that , and so we get