I'm confused... you can't have 2 decimals in a number.... anyway I'm going to help solve it as if the number was 10.5105
The formula for the area of a triangle is A = 1/2bh. So, if you were given the base and height and asked to find the area, you would multiply the base and height then divide by 2 (or multiply by 1/2, same thing). Here you have to do the opposite. Take the area, multiply it by 2, then divide it by the base.
10.5105(2) = 21.021
21.021/66 = 0.3185
h = 0.3185 ft
Hope this helped even if 0.3185 isn't the correct answer because 10.510.5 doesn't make sense lol.