Question 2 (ix)
Add:
ab - 4a, 4b - ab, 4a - 4b
ab - 4a, 4b - ab, 4a - 4b = ab - 4a + 4b - ab + 4a - 4b = - 4a + 4a + 4b - 4b + ab - ab = 0 + 0 + 0 = 0