keep in mind that, going against the current, the current's speed erodes speed from your regular speed, whilst if you're going with the current, the current's speed adds to it.
now, in this case, you row 5mph, going upstream you're only doing 3mph, whatever happened to the other 2mph? well, the current speed eroded them, meaning the speed of the river is 2mph.
now, going downstream with the current, your regular speed is 5mph, and the current is 2mph, since the current adds to yours, then you're going 5 + 2, mph.
I'm not completely sure but if you think your friend is 15 miles from it and you are four miles from your friend and you are actually closer to the store you would 15-4=11 then add 2 more miles because that is the distance from the grocery store so you should get 13 miles from home.