The thing you're looking for is the volume of the water in the tank at the beginning, so let's call it x. What you do know is the final volume and what was added or taken from the starting volume to get there, so, following the steps described in your task, the equation would look something like:
x+325L-270L+83L-157L=52L
Of course since you're looking for x, just throw all the non-x stuff on one side and x on the other, so to speak.
x=52L-325L+270L-83L+157L
x=71L