16x^3 = 4x^2 * 4x
so
4x * 1 2x - 4
-------------- - -----------------
4x * 4x^2 16x^3
4x 2x - 4
= ------------- - -----------------
16x^3 16x^3
4x - 2x + 4
= -------------------
16x^3
2x + 4
= -------------
16x^3
2(x + 2)
= -------------
16x^3
x + 2
= -------------
8x^3
hope it helps
