Maybe if you look at cycling opposite direction as the same as the total distance of Kester and Yurei, it should be easier to understand.
For 2 hours,
Yurei traveled 1u
Kester traveled 1u+3
If they traveled opposite direction, is the same as the total distance.
57km = Yurei + Kester = 1u + 1u + 3
57km =2u + 3
u = (57 – 3 )/ 2 = 27
In 2 hours, Kester traveled = 1u+3 = 27 + 3 =30km
Kester’s speed = 30/2 – 15km/hr ###