The short-run total cost function for a firm producing skateboards is TC= 18q^2-9q+72 where q is the number of skateboards per week. (1) What is the fixed cost? What is the variable cost? (2) Please calculate the average cost function for skateboards. (3) Please calculate the marginal cost function for skateboards. (4) Please show that when average cost reaches a minimum, marginal cost intersects average cost. Also, at what level of skateboard output does average cost reach a minimum? What is the average cost at this level of output?