What is the completely factored form of x3 – 64x?

X³ - 64x

by factoring out the x 
x³ - 64x = x(x² - 64)
              
by factorizing the bracket 
x³ - 64x = x(x² - 8x + 8x -64)
             = x (x - 8) + 8 (x - 8)
             = x (x + 8) (x - 8)

∴ x³ - 64x  =  x (x + 8) (x - 8)