What does it mean use doubles to find the sum 7+8.

none of the above.

for one of the equations to be used in the process, there has to be a term in the equation since squared is .

since there is only a term in the first equation, we cannot derive a from completing the square.

solving with completing the square

-2

7 - 9 = -2

reversed:

-2 + 9 = 7