4. For each of the following linear systems, write it into the form AX = b and express 6 as a linear combination of the column vectors of A. -x1 + x2 + 2x3 = 3 a. 2x1 + 6x2 5x3 = 2 -3x1 + 7x2 5x3 X1 - x2 + 4x3 = -1 | = 0 b. -2x1 + 4x2 3x3 = 0 3x1 + 6x2 8x3 = 0, - 2x2 + x3 = 0 . -x1+3x2 4x3