If the 3-year spot interest rate is 2.5%, the three-year forward rate for the following three years is 2.9%, and the forward interest rate starting in 6 years from now and ending in 9 years from now is 3.1%, calculate what the 9-year spot interest rate should be. Indicate the name of the theory that is the basis for determining these rates and what kind (type) of interest should be used in the calculations in this case)