Interpret the meaning of linear slope in a word problem, graph linear equations.

In business and economics, the rate of change of the cost is called the marginal cost. If the equation representing the cost, C, of producing n tires at Royal tires Co. a c an be written as C= mn + f, where f represents the company’s fixed costs, find the marginal cost of production if it is known that it costs $40,000 to produce 1,200 tires while it costs $60,000 to produce 2,000 tires.