The Hudson River flows at a rate of 3 miles per hour. A patrol boat travels 60 miles upriver, and returns in a total of 9 hours. What is the speed of the boat in still water?

distance upstream = (rate upstream) x (time upstream)
distance downstream = (rate downstream) x (time downstream)

You know the distance both upstream and downstream.

You don't know the rate, but if the boat's speed in still water is r, its upstream speed will be r-3, and its downstream speed will be r+3.

You don't know the time upstream or the time downstream, but you know the total time (upstream + downstream) was 9 hours.

That should give you three equations for your three unknowns (r, time_upstream, and time_downstream).