The two horizontal lines are parallel, so the smaller triangle is similar to the entire triangle.
That means the sides are proportional to each other. So pick two sides of the triangle and find the ratio between them. The ratio must be the same for both triangles.
(15/5) = (15+x)/8
Solve for x.
15*8 = 5(15+x)
120 = 75 + 5x
5x = 45
x = 9
