Question 144580
let x mph be the speed of the boat in still water then
(x+3)mph will be the speed of boat downstream
(x-3)mph will be its speed in upstream
now time taken by baot upstream t1=60/(x-3) and time taken by boat in downnstream t2=60/(x+3)
t1+t2=9
60/(x-3)+60/(x+3)=9
60{1/(x-3)+1/(x+3)}=9
x+3+x-3/x^2-9=9/60    taking LCM and dividing by 60 
2x/x^2-9=3/20
20(2x)=3(x^2-9)
40x=3x^2-18
3x^2-40x-18=0 solve this equation to get your answer