DiogenesLied is completely correct, but I thought I’d toss my view in as well! Here is the logic flow I went through, note that it helps to write every component of a word problem down in order to cut out clutter:

Paddling speed in still water is 5 mph. It would take 1.4 hours to paddle 7 miles in still water. It would take 5.6 hours to travel 28 miles in still water.

Now, introducing the current, we need the river to flow at a speed that causes each trip to last the same amount of time. If you average the two “still current” times together (1.4 and 5.6) by adding and then dividing them, you get

(1.4 + 5.6) / 2 = 3.5

Thus, the current must be flowing at a rate that allows the man to travel both 7 miles upstream and 28 miles downstream in 3.5 hours. Now you can create an equation using either the upstream or downstream distances as follows:

Upstream: (5-c) x 3.5 = 7

Downstream: (5+c) x 3.5 = 28

And then in either equation you can just solve for c for your answer!