so i got this problem below. what I see in my book is that WACC $=$ Wd$($r$\_$dt$)+$ Wps $($r$\_$ps$)+$ Ws $($r$\_$s or r$\_$e$).$ it seems the formula used here i can not find it$.$ could you please explain. thanks

Tropical Tours is considering an expansion of its operations, which will require the company to issue new debt and equity. Tropical$$s investment banker provided the following information about the cost of issuing new debt: Amount of Debt Issued Yield to Maturity, r d $$1$ $$450,0004\mathrm{.}5\%450,001750,0005\mathrm{.}8$ Over $750,0006\mathrm{.}5$ If tropical$$s capital structure consists of $60$ percent debt, what WaCC break points are associated with issuing new debt?