we will use the formula

t = S/v

where t is time, s is the distance and v is the speed.

let

c = the speed of the current

b = 25 mph the speed of the boat in still water

S = 21 miles the distance that the boat travels

t = 2.0 hours

(the time spent traveling 21 miles downstream with speed 25 + c) + ((the time spent traveling 21 miles upstream with speed 25 - c) = 2 hours

21 / (25 + c) + 21 / (25 - c) = 2

by solving this equation we find

c = - 10 is not a solution since c > 0

c = 10 mph is the solution

The speed of the current is 10 mph.