In an equation if both sides are square you can solve it by using roots for both sides.

$latex (x+5)^2=64

x+5=+-8

x=8-5=3 or x=-5-8=-13 $

But if one of sides isn’t a complete square you should complete it.

$latex x^2+4x=49

x^2+6x+9=49

(x+3)=+-7

x=-3-7=-10 or x=-3+7=54$

In cases like this you can complete the square by adding the square of half of b.(For example 6 in this case is b)