415.138;452.018;457.2;828.142
Use the equation for the area of a triangle:

where A = area, h = height of the triangle, and b = base of the triangle.
You know that h=18m and A=99

. Plug those values into the equation and solve for b, the base.
The base is 11m long.
7 times 3 equals 21, lol hopefully this helped