Please help me to understand the formula to finding the Present value of annuity immediate!

All i need help on is finding the formula for the present value !

the correct answer for e) $24850

Carlos needs a new roof for his house. He can pay cash, or pay by installments. Payments would be $144.68 bi-weekly (at the end of every period for 9 years. The interest rate is 7.500% compounded annually. a) This question deals with the x value of an annuity b) There will be x payments. The payment period is c) The payment amount is \$ d) The effective interest rate per period is x% e) The present/future value is $