Delta corporation has the following capital structure: cost (aftertax) weights weighted cost debt (kd) 5.2 % 10 % 0.52 % preferred stock (kp) 12.2 20 2.44 common equity (ke) (retained earnings) 7.1 70 4.97 weighted average cost of capital (ka) 7.93 % a. if the firm has $28 million in retained earnings, at what size capital structure will the firm run out of retained earnings? (enter your answer in millions of dollars (e. g., $10 million should be entered as "10" b. the 5.2 percent cost of debt referred to earlier applies only to the first $15 million of debt. after that the cost of debt will go up. at what size capital structure will there be a change in the cost of debt?