For the purpose of this puzzle assume that the UTM coordinates are points on an ideal flat plane. You are provided with these two points in UTM coordinates, easting then northing.
A = 612502 4176376
B = 612502 4173976
The geocache is located at point C. Point C is 2km from point A and the bearing from point C to point B is 125.469889191 degrees from true north.
You can validate your solution with Certitude
You can validate your puzzle solution with certitude.
A similar but more challenging problem is Trigonometry on the Ellipsoid.