(x+2)^3-(x-2)^3
= {(x+2) - (x-2)} { (x+2)^2+(x+2)(x-2)+(x-2)^2 }
= {(x+2 - x+2)} { (x^2+4x+4)+(x^2-4)+(x^2-4x+4) }
= 4 {2 (x^2+4)+(x^2-4) }
= 4 {2x^2+8+x^2-4 }
= 4 {3x^2+4 }
= 12x^2 + 16