In $\triangle ABC$, $\angle ABC = 1$ right angle. If $AB = 9$ and $BC = 12$, then $AC =$

The right angle is at $B$, so $AC$ is the hypotenuse: $AC = \sqrt{AB^2 + BC^2} = \sqrt{9^2 + 12^2} = \sqrt{81 + 144} = \sqrt{225} = 15$.