The most general proccedure for this type of problems is the following:
For a quadratic function of the following form
; where b and c are real numbers.
1) Identify the value that accompanies the x, that is, the value "b"
2) Divide the value of b by 2 and then square it:

3) Add and subtract in the function the value of

4) Write the complete function of the form
For the function presented
the result is as follows:
1) The value of b is 12
2)

3)

4)

Finally the function is:

The zero pair that must be added is 36 and -36