MC is given by ........

Explanation:

MC is given by Slope of TC. Slope tells us how (total) costs change when we change output/quantity produced. The slope of the total variable cost curve is marginal cost, as well. The relation between total cost and marginal cost is but another in the long line of applications of the total-marginal relation. The slope of the total cost curve (and total variable cost curve) is marginal cost.