Maria,
(3x-2)4
(3x-2)2 = 9x2-12x+4
So (3x-2)4 = (9x2-12x+4)2
Then
(9x2-12x+4)(9x2-12x+4) =
9x2(9x2-12x+4) - 12x(9x2-12x+4) + 4(9x2-12x+4) =
Distribute
81x4 - 108x3 + 36x2 -108x3 + 144x2 - 48x + 36x2 - 36x + 16 =
and combine terms
81x4 - 216x3 + 216x2 - 81x + 16
