There is a small triangle on the left side. It has a 6-m side and a 5-m side.
There is a large triangle which has a 13.5-m (= 6 m + 7.5 m) side and a side measuring x.
The triangles are similar, so the lengths of the sides are in proportion.
5/6 = x/13.5
6x = 5 * 13.5
6x = 67.5
x = 11.25
Answer: x = 12.5 m
