Answer:
15 feet
Explanation:
The distance from the foot of the ladder to the person is 6 feet. The person is also 6 feet. Therefore that mini triangle's length and width are both 6 feet.
The distance from the foot of the ladder to the wall is 15 feet. This is now a simple ratio. The length of the mini triangle is 6 to length of the larger triangle, 15. The height ratio is the mini triangle is 6 to the bigger triangle, x.
The height of the bigger triangle is the height of the wall. (Since the two are similar, then we can automatically deduce the height is 15.) But if you wanted to work out the ratio above ^^ you would multiply the 6 diagnol to the 15 and then divide by the last 6 to get 15.