Please help!! I keep getting the questions wrong I do not know why. Please Help!!

Thunderhorse Oil. Thunderhorse Oil is a U$.$S$.$ oil company. Its current cost of debt is $7\mathrm{.}40\%,$ and the $10-$year U$.$S$.$ Treasury yield, the proxy for the risk$-$free rate of interest, is $3\mathrm{.}10\%.$ The expected

return on the market portfolio is $8\mathrm{.}60\%.$ The company's effective tax rate is $40\%.$ Its optimal capital structure is $65\%$ debt and $35\%$ equity.

a$.$ If Thunderhorse's beta is estimated at $0\mathrm{.}80,$ what is Thunderhorse's weighted average cost of capital?

b$.$ If Thunderhorse's beta is estimated at $0\mathrm{.}40,$ significantly lower because of the continuing profit prospects in the global energy sector, what is Thunderhorse's weighted average cost of capital?

a$.$ If Thunderhorse's beta is estimated at $0\mathrm{.}80,$ what is Thunderhorse's weighted average cost of capital?

$\%($Round to two decimal places.$)$