Hello. I found a different answer as to what the previous user got. I’ll show you my work and you can determine which one is right.
When you see two triangles, you can guess we’re going to deal with proportions. I’m going to set up a proportion between the long side of the big triangle and the long side of the little triangle
15/7
Ok...we’re making progress. We can make another proportion with height h and the “height” of the big triangle, 9
9/h
Since both of these are proportions, we can set them equal to each other
(9/h) = (15/7)
Cross multiply...
63 = 15h
h = 4.2 ft