Can you show us what you have tried?
Lets give a few hints: V(t) is the volume of the tank in gallons. V(0) = 50. You lose 2 gallons a day which means you lose 2*t gallons for every t days. Now you add give gallons every third day which means you add 5 gallons every 3 days, or if you have n days you add n/3 - n % 3 where n % 3 is the remainder you get from dividing 3 into n.