To complete the square we add a constant (23) so that the constant on the left is (18/2)² = 81.
x² -18x +58 +23 = 23
(x -9)² = 23 . . . . . . . . . . . rewrite the left side as a square
x -9 = ±√23 . . . . . . . . . . . take the square root
x = 9 ±√23 . . . . . . . . . . . .add 9
The answer corresponds to selection ...
D. x = 9 ±√23