let's look at an example
x^2 + 6x + 3 = 8
Take the "b" coefficient of the quadratic equation, which is 6 and divide in half and square it.
6/2 = 3
3^2 = 9
9 is what we need to be the constant on the left side not 3. We can add 6 to the 3 to make 9 ... but what ever you do to the left side must also be done to the right side to keep the equation the same. .
think of it in simple terms: 5 = 5
if you add 3 to the left but not the right it is not valid 5+3 does not equal 5 anymore
x^2 + 6x + 3 + 6 = 8 + 6
x^2 + 6x + 9 = 14
( x + 3)^2 = 14