x^3=12x+16
x^3-12x-16=0
x^3-4x-8x-16=0
x(x^2-4)-8(x+2)=0
x(x+2)(x-2)-8(x+2)=0
(x＋2)(x＋2)(x-4)=0
x1=-2
x2=-2
x3=4