substitue two equations on the right (for Vf and (Vf/Vi)^Cp/Cv) into the equation for the ratio of work, and derive until the second line of the equations that is shown in the picture

3.20 Show that for an ideal monatomic gas expanding adiabatically against the constant external pressure of a mechanical reservoir (sur) to a final equilibrium state where Psys = Psur, the ratio of work compared with reversible adiabatic expansion over the same volume limits is given by the first equality in Equation 3.60, that is, Wire ad _ -Psurr (V-V) Vi= y ( V3p 5 Psurr +2 We ad - 2p sur 1 3p: 5 p surr +2 Cy+nR VI V PS +2 Psurr -2(V;-) V (5V, -2V) V bol -20)