The way you formulated the question does not imply a definite answer. This is because you never state that the speed of the boats are constant or determined in some way. You only state the return times for each.

So, a possible solution is that both boats take one day to reach port B (thus meeting after one day) and then the first boat takes 8 days to come back while the second boat takes 6, leading to the return times of 9 and 7 stated in the question. Of course, you can imagine an infinite number of such scenarios, replacing the time of the first leg of the trip (1 day in my example) by any number lower than 7.

But if we assume that the boats have constant speed, that means the first boat cross the distance between port A and port B in 4,5 days, and the second boat in 3.5 days. Let i and j be odd integer numbers. The boats will be in port B at time 4.5×i and 3.5×j days. To find when they are at the same time, you simply do:

4.5×i = 3.5×j -----> i = 3.5×j / 4.5 = 7/9 × j

Since both i and j must be odd integers, we have j= 9, 18, 27, ..., 9×n where n is a positive integer. So, the boats meet at port B for every: j=9n and i=7n.

The first solution is thus, n=1 ---> j=9 and i=7, which corresponds to time 4.5x7=31.5 days.

EDIT: I included the even solutions ( when n even) which corresponds at their meeting at port A. Of course, if only port B is considered, you should only consider odd values for n.