What is the marginal profit gain (per unit) for the linear function which is Profit (x) = Revenue (x) - Total Cost (x), x being the volume? Where do the lines for profit(x) and contribution(x) respectively intersect with other lines like cost or sales revenue?

a) in general and

b) in this particular example with these data given

- Cost(x) = 15 + 3 * x

- Revenue(x) = 4 * x