Find how long it takes to fill 1/9 of the tank by dividing 2 minutes by 5 (since 5/9 is already filled). 2 minutes is 120 seconds. 120/5=24 seconds.
Now multiply the time it takes to fill 1/9 of the tank (24 secs) by 9 to get the time needed to fill 9/9 aka the entire tank. 24*9=216 seconds.
Convert 216 seconds to minutes. 216/60=3.6 minutes needed to fill the whole tank.