Let s represent the speed of the boat in still water

D = r*t  OR t = D/r

Question states*** (note round trip took 1.75hr)

 12/(s+3) + 12/(s-3) = 1.75hr

Solving for x

 12(s-3) + 12(s+3) = 1.75(s+3)(s-3)

    24s = 7/4(s^2-9)

     7/4s^2 - 24s - 63/4 = 0

       7s^2 - 96s - 63 = 0

  |discarding the negative solution

 s = 14.342 kmph