Essentially, we are trying to find the missing constant term of (remember that we are subtracting due to the negative sign in front of the second term). Let's expand this to see what we can work with:
Set up
FOIL and simplify
Now, we know the second term is , so let's set the second term in the polynomial we just found equal to :
Set up
Divide both sides of the equation by
Divide both sides of the equation by 2
We have found . We know the missing constant term is , according to the polynomial we found earlier. Thus, the missing term is: