This is because Gavin representing "x" loses 3 marbles, which is shown by the -3 in the parenthesis, is half of what Cindy has, or as the problem says "Cindy has double the marbles that Gavin has." These two together result in y=2(x-3)
To find angle y you subtract the two angles form 180 degrees because it is a straight line. Then you subtract angle y and 72 from 180 because a triangle adds up to 180 degrees and you get angle x.
Hope this helps, ask if you have any more questions!
= -2
