Answer
$w_s=311.8\ Btu/lbm$
Work Step by Step
From tables A-4E to A-6E:
Inlet ($P_1=800\ psia, T_1=900°F$): $s_1=1.6413\ Btu/lbm.°R,\ h_1=1456.0\ Btu/lbm$
Outlet ($P_2=40\ psia, s_2=s_1$): $x_2=0.9725,\ h_2=1144.2\ Btu/lbm$
From the energy balance:
$\dot{m}h_1=\dot{m}h_2+\dot{W}_s$
$w_s=h_1-h_2$
$w_s=311.8\ Btu/lbm$
