can someone explain to me how they got $\displaystyle (4x - y)$ in the numerator?

problem: $\displaystyle ( 64x^3 - y^3 ) / ( 4x^2 - 33xy + 8y^2 )$

first step: $\displaystyle [ ( 4x - y ) ( 16x^2 + 4xy + y^2 ) ] / [ ( 4x - y ) ( x - 8y ) ]$

I know they used the perfect cube formula, but why use 16 and 4. and where did the $\displaystyle (4x - y)$ come from?