How do I know what values to enter into the appropriate variables?

Question

The questions is "The owner of a clothing manufacturer finds that the total cost of producing 300 of a certain brand of shirts is $2,000, and the marginal cost when 300 shirts are produced is $15 per shirt. Using these numbers, the total cost of producing 302 shirts is approximately [BLANK]"

I'm know that I use the linear approximation L(x) = f(a) + f '(a) (x - a), but I'm not sure what each variable means or what goes to what.

Answer 1

L(x) = f(300) + f^'(300) (x-300) = 2000 + 15 * (302 -300) = $2030.