For this case we have the following equation: f (x) = x ^ 2 + bx-49 Deriving we have: f '(x) = 2x + b We match zero: 0 = 2x + b We clear x: x = -b / 2 The axis of symmetry is at x = 8, therefore: x = -b / 2 = 8 Clearing b: b = -2 * (8) b = -16 Answer: the value of b is: b = -16