6. Suppose you have a lemonade stand, and when you charge $2 per cup of lemonade you sell 100 cups. But when you raise your price to $4 you only sell 50 cups. Write an equation for the number of cups you sell as a function of the price you charge. Denote "C" for number of cups, and "P" for the price you charge. Assume the function is linear. Show your work.