Since this is a linear function, you have to figure out the slope and the y-intercept. The y-intercept is easy. It's the place where the line crosses the y-axis. that's at y = 25. To find the slope, you need to find change in y divided by change in x. You can see at that x = 0, the line is at (0, 25). At x = 20, the line is at (20, 30). Therefore, the slope is (30-25)/(20-0) = (5/20) = 1/4.