Factor the expression completely

X^4 - 16

Note: (x) (y) means x times y

this is the difference of 2 perfect squares

(x^2) - (y^2) = (x+y) (x-y)

so

x^4-16 = (x^2-4) (x+4)

x^2-4 is also the difference of 2 perfect squares

x^2-4 = (x-2) (x+2)

the answer is

(x-2) (x+2) (x+4)