There are 2 overlapping triangles here. If the vertical lines are parallel, then these triangles are similar, and corresponding sides occur in a fixed ratio. In particular, we have
2/3 = (2 + x) / (3 + (x + 7))
Solve for x :
2/3 = (2 + x) / (10 + x)
2 (10 + x) = 3 (2 + x)
20 + 2x = 6 + 3x
14 = x
