x8 comes from -3x2, which needs to be raised to the fourth power: (-3x2)4=81x8.
Take (a+b)7, where a and b are any quantities. The expansion is:
a7+7a6b+21a5b2+35a4b3+35a3b4+...
35a3b4 is the term we need to compute when a=2 and b=-3x2.
a3=8, b4=81x8, as we saw earlier.
So the coefficient is 35×8×81=22680.