What formula is used in this question? Please give me that formula.

Problem 7.22. The one-year LIBOR rate is 10% with annual compounding. A bank trades swaps where a fixed rate of interest is exchanged for 12-month LIBOR with payments being exchanged annually. Two- and three-year swap rates (expressed with annual compounding) are 11% and 12% per annum. Estimate the two- and three-year LIBOR zero rates when LIBOR discounting is used. The two-year swap rate implies that a two-year LIBOR bond with a coupon of 11%sells for par. If R, is the two-year zero rate 11/1.10+111/(1+R)? = 100 so that R, = 0.1105 The three-year swap rate implies that a three-year LIBOR bond with a coupon of 12% sells for par. If R, is the three-year zero rate 12/1.10+12/1.1105 +112/(1+R2) = 100 so that R, = 0.1217 The two- and three-year rates are therefore 11.05% and 12.17% with annual compounding