Paul makes y doughnuts at his shop every day. The expression 13y - 6 = 78 represents how much Paul made on Wednesday where y represents the cost of the sprinkles on the doughnuts. How much in total were the sprinkles?
First, you must add 6 to both sides to get 13y alone. Whatever you do to one side you have to do to another. In order to fully get y alone, you must divided both sides by 13 because 13 if the coefficient of y.
This would leave you with y = 7. I made a profit of $7 just from the sprinkles!