Verified Answer :

HunterX

he number that must be added is four because (x+2)^2=x^2+4x+4

x^2 + 4x
the number to be added is found by dividing the constant of x by 2
 and squared the number
so # to be added =  ( 4/2)²  =  4
the perfect square trinomial is
x² + 4x + 4    =    (x + 2 )²    which is a perfect square trinomial
I hope this help you