A company has estimated cost of debt and equity capital for various proportions of debt in its capital structure with these estimates how would one determine an optimal capital structure, how would one begin to formulate this answer using what exactly

% Debt                                  After-tax cost of debt                         Cost of equity

0%                                            -                                                       13.0%

10                                             5.4%                                                 13.3%

20                                            5.4%                                                   13.8%