12. Let A 12 6 8 and B 5 7 9 10 Determine Matrix Multiplication 71 A 3A 21 2B (e) 2A - 3B () 13. Let all matrices in this exercise matrix with\f5. (a) In the matrix A for the refinery model in Example 1, state in words what the numbers in entries a, and entries a,, represent. What do the numbers in the third column of A represent? (b) Suppose that refinery 3 is modernized and its output for each barrel of oil is doubled. What is the new matrix of coefficients? (c) In Example 1 of Section 1.4 we discussed the situation where re- finery 3 broke down and was out of service. In this case, what is the matrix of coefficients?10. Consider the system of equations 2x, + 3x, - 21, = Sy, t 2). - 3y, + 200 2y. 350 (a). Write this system of equations in matrix form. Define the vectors and matrices you introduce. (b) Rewrite in matrix form with all the variables on the left side (and just numbers on the right) uters but