For this question:

Eva catches two kinds of fish - salmon and trout. Assume salmons and trouts come independently at arrival times of Poisson process with rate 1 per hour and 2 per hour.

Given that Eva caught 3 salmons in the first hour, what is the expected number of fishes she could have caught for the first two hours?

the answer is:

can someone please explain how to get the second line? like is there some sort of formula to get that?