Hello there, please can someone help me to solve part (d)

The Nolan Corporation finds it is necessary to determine its marginal cost of capital. Nolan's current capital structure calls for 30 percent debt, 20 percent preferred stock, and 50 percent common equity. Initially, common equity will be in the form of retained earnings (Ke) and then new common stock (Ke). The costs of the various sources of financing are as follows: debt (after-tax), 5.2 percent; preferred stock, 7 percent; retained earnings, 10 percent; and new common stock, 11.2 percent. a. What is the initial weighted average cost of capital? (Include debt, preferred stock, and common equity in the form of retained earnings, Ke.) Note: Do not round intermediate calculations. Input your answers as percent rounded to 2 decimal places. b. If the firm has $27 million in retained earnings, at what size capital structure will the firm run out of retained earnings? Note: Enter your answer in millions of dollars (e.g., $10 million should be entered as "10"). d. The 5.2 percent cost of debt referred to earlier applies only to the first $42 million of debt. After that, the cost of debt will be 7.2 percent. At what size capital structure will there be a change in the cost of debt? Note: Enter your answer in millions of dollars (e.g., $10 million should be entered as "10"). e. What will the marginal cost of capital be immediately after that point? (Consider the facts in both parts c and d.) Note: Do not round intermediate calculations. Input your answer as a percent rounded to 2 decimal places