Let x represent as the number of cards in 1 sheet.
So... 7x means 7 times the number of cards that go in a sheet.
7x+4 means that there are four cards left.
7x+4 = 130
7x+4-4 = 130-4 (if you do something to one side, you have to do the exact same thing to the other side)
So... 7x = 126
7x/7 = 126/7
x = 18
when you divide both sides by 7 you get x by itself.
That means 18 trading cars go in a sinle sheet with only four cards going in the eighth sheet.