I have no idea how to do this one.
All help is greatly appreciated.
(not sure if this is right section)
Let Vs be the velocity of the slower train, and Vf is that of the fast train.
The first statement leads you to
b/Vs - b/Vf = a. Or
1/Vf - 1/Vs = a/b .....(1)
The second statement leads you to
Vf*c - Vf*c = 1 km Or
Vf - Vs = 1/c .........(2)
Vf = Vs + 1/c
Substitute this value of Vf in equation (1) and solve the quadratic to find Vs.