Split the original triangle down the middle to form two congruent triangles. Focus on one of the two smaller triangles. With 36 degrees as the reference angle, we have 6.25 as the adjacent side and R as the hypotenuse
Use the cosine function
cos(angle) = adj/hyp
cos(36) = 6.25/R
R*cos(36) = 6.25
R = 6.25/cos(36)
R = 7.72542485937369
Multiply both sides by 2 to get
R = 7.72542485937369
2*R = 2*7.72542485937369
2*R = 15.4508497187473
So that rounds to 15.45
Answer: 15.45 feet