Answer: 17 and 23.
Explanation: Let the rate of the slower cyclist be x.
After 3 hours, the total distance between the both will be of:
3x (distance from the slower cyclist from the starting point) + 3(x + 6) (distance from the faster cyclist from the starting point) = 120.
Solving by x, 17 = x. Since x is the rate of the slowest cyclist, x + 6 = 23, is was the rate of the faster cyclist.