Assume that MM's theory holds with taxes. There is no growth, and the $40 of debt is

Question:

"Assume that MM's theory holds with taxes. There is no growth, and the $40 of debt is expected to be permanent. Assume a 40% corporate tax rate.
a. How much of the firm's value is accounted for by the debt-generated tax shield?
b. How much better off will UF's a shareholder be if the firm borrows $20 more and uses it to repurchase stock?"