We have...........
(3x + 4)3 - (3x - 4)3 = [(3x)3 + (4)3 + 3 × 3x × 4 × (3x + 4)] - [(3x)3 - (4)3 - 3 × 3x × 4 × (3x - 4)]
= [273 + 64 + 36x (3x + 4)] - [273 - 64 - 36x (3x - 4)]
= [27x3 + 64 + 108x2 + 144x] - [27x3 - 64 - 108x2 + 144x]
= 27x3 + 64 + 108x2 + 144x - 27x3 + 64 + 108x2 - 144x
= 128 + 216x2
∵ (3x + 4)3 - (3x - 4)3 = 128 + 216x2
