dyga

Which part are you having trouble with? I’ll go ahead and provide what is hopefully a helpful explanation for each column.
D/D+E is just given for you to solve and I’ll refer to as D/A below.
Beta: You are given the unlevered beta of 0.9. Using the D/A to get to D/E by doing [D/A / (1D/A)] and then you can solve for Beta by using the formula = Bu * [1+D/E*(1tax)]
Cost of debt: they give you the base rate of 4.5% which you apply a spread to as given in the table above for different levels of debt. 100bps = 1% so as an example at a D/A of .4 the spread is 300bps so 4.5%+3%=7.5%
Cost of equity: Use CAPM. You just found the levered betas and they give you the risk free rate and market risk premium. Re=Rf+Beta*MRP
WACC: Use all the other inputs you just found: WACC=Re*(1D/A)+Rd*D/A*(1tax)
The optimal capital structure is at 30% debt because WACC is minimized