Answer:
answer is subtract the 180-120=80
I think your answer would be 4,000
ANSWER
We need to add half the coefficient of
all square.

EXPLANATION
Given the equation,
.
We compare to the general equation,

This means that;

To complete the square we add, 
That is
.
Hence the number to be added to both sides is
.
Note that before we complete the square, the coefficient of the term in
must always be 1.