Question 962688
Let 'x' be the speed of the boat in miles per hour in still water

During upstream relative speed of the boat will be (x-3) miles per hour

During downstream relative speed of the boat will be (x+3) miles per hour

total time = time taken (upstream)+ time taken (downstream)

3= distance/(upstream speed)+ distance/(dowmstream speed)

12/(x-3)+12/(x+3)=3

=x^2-8x-9=0

x=9 miles /hour