Question 363778
Distance = Speed * Time 
Let s = speed of motorboat in still water
    t = distance / speed

    8/(s+2) + 8/(s-2) = 3
Multiply both sides by (s+2) (s-2)
     8 (s-2) + 8 (s+2) = 3(s+2) (s-2)
         8s-16 + 8s+16 = 3 (s^2-4)
                   16S = 3s^2 -12
                       
 Subtract 16s from both sides
                     0 = 3s^2 -16s -12
                       = (3s+2) (s-6)

            s-6 =0
            S    = 6 

The speed of the motorboat is 6 miles per hour.