Answer:
16
Step-by-step explanation:
The constant in a perfect square trinomial is the square of half the coefficient of the linear term.
(x +a)² = x² +2ax +a²
For 2a=-8, a=-4 and a² = 16. The constant 16 must be added:
x² -8x +16 = 7+16 . . the left side is a perfect square trinomial
(x -4)² = 23 . . . . . . . the left side is a perfect square
