I think they had asked from the time from t=0, from the point when collision occurs.
1. Detecting collision = propagation delay / 2 i.e 2 micro seconds.
(As jamming signal travels half of the link to reach source X)
2. Waiting for one time slot of 2* propagation delay i.e 4*2 micro seconds = 8 micro seconds.
(After jamming signal arrives backoff algorithm applied and k value is evaluated randomly)
3. Time taken for transmitting packet onto the link and waiting for its last bit to reach station Y i.e transmission delay + propagation delay = 10 + 4 micro seconds = 14 micro seconds.
Adding 1, 2, 3 we gets total time as 24 micro seconds.
Please correct me if i am wrong.