a motorboat whose speed in still water is 18 kilometre per hour takes 1 hour more to go 24 km upstream than to return downstream to the same spot.find the speed of the stream..

Why there minus symbol is used instead of plus....

Dear student, 
speed of boat upstream is given as (18-x)km/hr.
speed of boat downstream is given as (18+x)km/hr.
Also given that boat takes 1 hour more to go upstream than going down stream. 
That's why time taken by boat for upstream is greater than going downstream. 
so,  we need to subtract time for downstream from time for upstream and that will be equal to 1 hour. 
Regards.