You answered these questions for me recently... On #2a and #2b, when you calculated the cost of equity, and the WACC, why did you know use the tax

You answered these questions for me recently... On #2a and #2b, when you calculated the cost of equity, and the WACC, why did you know use the tax rate (40%) in your equations? since the problem states that it is "an MM world with taxes"...