If f(x) = 6x , then f(2) = 6(2) or f(2) = 12... simply substitute the argument in for the variable in your expression on the right.
The cattle train
speed - 36.5 km/h
time - x h
the passenger train
speed - 29.2 km/h
time - (2.4+x) h
The cattle train caught up to the passenger train so the distance they travelled is the same.
Distance is speed times time.

The passenger train travelled for
12 hours before the cattle train caught up.
5x + 6
6 more than 5 times a number
Hope this is correct!
The answer is D
Hope this helps!