1) Use the info given...
X holds 600 gallons more than Y...X=Y+600
When 100 gallons are taken from each tank, X has 3 times more...X-100= 3(Y-100)
You end up with 2 equations:
Set them equal Y+600 = 3Y-200. Solving for Y, you get Y=400, plug it back in to get X= 1000. 1000+400=1400.