Multiply:  x2 + 4xy + 16y2 x - 4y

Strategy:
Multiply EVERY term above by EACH term below
Arrange these products in columns by degree.
Add the coefficients in each column.
Some sums may be ZERO.

Multiply:  x2 + 4xy + 16y2 x - 4y -4x2y - 16xy2 - 64y3 x3 + 4x2y + 16xy2          x3                       - 64y2

Answer is: x3 - 64y2