Solution:f(x) = x² − 9x + 22⇒ y = x² − 9x + 22
⇒ x² − 9x + (22−y) = 0
∴ x = [−(−9) ± (−9)² − 4 (22−y)] / 2
⇒ f^(1) (y) = [9 ± √(81 − 88 + 4y)] / 2
⇒ f^(1) (y) = [9 ± √(4y − 7)] / 2
⇒ f^(1) (4) = [9 ± √(4 * 4 - 7)] / 2
= 9 ± √9 / 2 = (9 ± 3) / 2 = {3, 6}