Please help me with this calculus question!

Question

The cost in dollars of producing x units of a particular brand of notebook is C(x) = x²- 400.

1. Find the average rate of change of C with respect to x when the production level is changed from x = 100 to x = 102. Include units in your answer.
2. Find the instantaneous rate of change of C with respect to x when x = 100. Include units in your answer.

Answer 1

For part 1, the average rate of change is the change in C(x) divided by the change in x.

For part 2, the instantaneous rate of change is found by finding the derivative of the function, then using the value of x in question.