This is by similar triangles:
You can actually pull out two similar triangles from the figure. One large, and the other small, but they are both similar.
The large triangle has sides: 62, 78, 98
The smaller triangle inside has sides: 2x, 39, 49
By similar ratio:
(2x/62) = (39/78)
2x/62 = 1/2
4x = 62
x = 62/4
x = 31/2
x = 15.5
Option A.
