Rewrite the left hand side as the square binomial.
X^2+4x+4=16
(X+ )^2=16
well (x+ a)^2 = x^2 + 2 a x + a^2 = x^2 + 4 x + 4
so
2 a = 4
AND
a^2 = 4
there is only one number like that, 2
X^2+4x+4=16
(X+ )^2=16
so
2 a = 4
AND
a^2 = 4
there is only one number like that, 2