There are two possible answers depending on how the 9 ft is interpreted. If the 9 ft is the distance along the ground between Charles and the mirror, the calculation is:
6/9 = h/63 and h = 42.
If the 9 ft is the distance from Charles' eye to the mirror, then the distance along the ground is sqrt(81-36) = 6.7 ft
6/6.7 = h/63 and h = 56 ft.