P4 revision note

sir I have already went through the revision not. the following questions I do not understand when I looked at 1. in what situations can we use the risk free rate to calculate the cost of debt? in the revision note, some of the places used the risk free rate tot get the cost of debt directly. 2.Example, this is question is attached in the revision note Company is financed as follows: Equity: 10M $0.50 shares, quoted at $1.20. Debt: $6M 9% irredeemable debenture (unquoted) The ? of a share in the company is 0.85 The market return is 20%, risk free rate is 12% Corporation Tax is 30%; Ignore Income Tax. Calculate: (a) the market value of debentures (b) the WACC of the company in the answers I do not understand '(No ? given for debenture, so assume risk free therefore investor require 12%) M.V. Debt = 9/12% = $75 p.c.' what does it mean? also when calculate the cost of debt, use 12%*(1-30%)? what situations can I use this method to get the KDat? thanks!