The graph represents revenue in dollars as a function of greeting cards sold. A coordinate plane showing Greeting Card Revenue, Number of Cards Sold on the x-axis and Revenue in dollars on the y-axis. A line starts at (0. 0) and passes through (2, 8), (4, 16), and ends at (5, 20). Which equation represents the function shown on the graph? y = x y = x y = 2x y = 4x