basically its asking you to make an equation out of the given situation and relate it to the graph
in this situation, u can see that the constant is 17.89 as that is the activation fee and the gradient would be 3 for every gigabyte. we sub y as the cost and x as the gigabyte. so the gradient y/x would be cost/gigabyte which is 3
so your graph would have an equation of y=3x+17.89