How do i find the isentropic exit enthalpy, h$2\text{'},$ which is last variable to solve for the turbine efficiency on this turbine problem? I solved for the inlet properties at state $1$ using superheated steam tables, and got h$1=1454\mathrm{.}3$Btu$/$lbm and s$1=1\mathrm{.}626$ Btu$/$lbm$-$degree R$,$ at P$1=900$ psia & T$1=900-$degree F$.$ Then for Outlet properties at state $2,$ at P$2=96$ psia & T$2=450-$degree F$,$ found actual exity enthalpy, h$2=1254\mathrm{.}49$ Btu$/$lbm$,$ and under isentropic expansion, s$1=$ s$2\text{'}=1\mathrm{.}626$ Btu$/$lbm$-$degree R$,$ again in superheated steam tables. But now i am stuck on how to find the isentropic exit enthalpy, h$2\text{'}?$

THERMAL AND FLUID SYSTEMS PRACTICE EXAM

$70.$ The turbine efficiency from State $1$ to State $2$ is most nearly:

A$.75\%$

B$.79\%$

C$.82\%$

D$.87\%$