Find the probability that the train arrives late on exactly 2 days given a certain schedule and historical data.

Mathematics: Given that ∫(4 to 1) f(x) dx = 2, ∫(4 to 3) f(x) dx = 6, and ∫(7 to 1) f(x) dx = 9, find ∫(7 to 3) f(x) dx.
a) 3
b) 4
c) 5
d) 6