Answer:
a. Mullineaux's WACC = 0.60*12 + 0.05*5 + 0.35*7*(1 - 0.35)
WACC = 7.2 + 0.25 + 0.35*7*0.65
WACC = 7.2 + 0.25 + 1.5925
WACC = 9.0425%
WACC = 9.04%
b. After tax cost of debt = 7*(1 - 0.35)
After tax cost of debt = 7*0.65
After tax cost of debt = 4.55%
So since after tax cost of debt of 4.55% is less than the preferred cost of 5%, company should use debt in its capital structure.