I would use the Law of Sines to solve this problem.
The angle next to the 58 degree angle is 122 degrees. The side opposite this angle is 3.72 km.
The angle we need to find is at the cave. The side opposite this angle is 2.60 km.
(Sin 122/3.72) = (Sin x/2.60)
Cross multiply to solve.
3.72 Sin x = 2.60 Sin 122
Divide both sides by 3.72
Sin x = (2.60 Sin 122)/3.72
Sin^-1 x = (2.60Sin122)/3.72
x = 36 degrees
