Solve the system of equations by substitution. Sx+ y=0 2x+ 3y = -7 x = 6, y = -6; (6, -6) x = -6, y = 6; (-6, 6) x = 7, y = -7; (7, -7) x = -7, y = 7; (-7, 7) Perform the row operation(s) on the given augmented matrix. (a) R 2 = -2r 1 + r 2 (b) R 3 = -3r 1 +r3 (c) R 3 = 5r 2 + r 3 [1 -3 [1 -3 -5 2 1 12 1 0 -51 -21 5 [1-3 -5 2 0 1 12 1 0 9 79 5 [1-3 -5 2 0 1 12 1 0 -9 49 5 [1-3 -5 2] 0 11 12 1 0 15 31 1