you have two walls that are 10 by 7.5 so 75 feet ^2 in area each
you have another two walls that are 14 by 7.5 so 105 feet^ 2 each
so all of that together is 360 feet^2 to cover for one coating, so you have 720 feet^2 to cover for 2 coats on the walls. so take that and divide it by the area that one gallon would cover, then round up giving you how many gallons you would have to buy
for the ceiling you do the same, the area is 140 and you only need one coat