Remember, the general rule is if A is (m x n) and B is (n x p), then AB is (m x p).
Since A is (3 x 5) and AB is (3 x 1), we must have that B is (5 x 1). (No. of rows of B must be 5 in order for B to be compatible with A in the product AB, and no. of columns of B must be 1 in order that the no...