On the return trip the boat covers the same distance x in a time of 2.5 hours, now moving with a speed of v + 3 according to a landlubber.
(I've fiddled with the last equation to make it slightly more useful.)
Now, solve the top equation for (rather than for just x, simply for convenience. If you are unsure about this step, solve it for x.) Since this is already done, this is easy!
Now insert this into the second equation:
So v = 9 mph.