Answer: add this flight because marginal revenue exceeds marginal costs.
Explanation:
Since the total cost of the flight would be $1,100, of which $800 are fixed costs already incurred, then the variable cost in this case will be )$1100 - $800) = $300.
Since the expected revenues from the flight are $600, thus implies that the total revenue exceeds total variable cost and therefore Dash should add the flight because total revenue is more than total variable cost and the marginal revenue exceeds marginal costs.