Matrices discrete mathematics?

you know that B is a 3 x 4 = { b_ij} so that AB has [2 b_11 - 2 b_21 + 4 b_31 = 12] for the 1st entry (1,1) and -b_11 + b_21 + b_31 = 8 for (2,1) entry...similar for the other entries....this yields b_31 = 28/6 and -b_11 + b_21 = 10/3, thus no unique solution will exist...b_11 = 1/3 & b_21 = 3 or {0,10/3} also works...I got the bottom row of b to be [ 28/6....19/6...82/6...37/6]..note: the top row of B could be all zeros!!!