How do I know when to use WACC vs MM II? For example:

Shadow Corp. has no debt but can borrow at 5.4 percent. The firms WACC is currently 9.3 percent and the tax rate is 22 percent.

a.	What is the companys cost of equity? (Do not round intermediate calculations and enter your answer as a percent rounded to 2 decimal places, e.g., 32.16.)
b.	If the firm converts to 35 percent debt, what will its cost of equity be? (Do not round intermediate calculations and enter your answer as a percent rounded to 2 decimal places, e.g., 32.16.)
c.	If the firm converts to 55 percent debt, what will its cost of equity be? (Do not round intermediate calculations and enter your answer as a percent rounded to 2 decimal places, e.g., 32.16.)
d-1.	If the firm converts to 35 percent debt, what is the companys WACC? (Do not round intermediate calculations and enter your answer as a percent rounded to 2 decimal places, e.g., 32.16.)
d-2.	If the firm converts to 55 percent debt, what is the companys WACC? (Do not round intermediate calculations. Enter your answer as a percent rounded to 2 decimal places, e.g., 32.16.)