HA:=5570;
HB:=6820;
n:=1;
TA:=77.4;
TB:=90.2;
R:=8.31;
fA[T_]:=Exp[(HA/(n*R))*((1/T)-(1/TA))];
fB[T_]:=Exp[(HB/(n*R))*((1/T)-(1/TB))];
xc[T_]:=fB[T]*((1-fA[T])/(fB[T]-fA[T]));
xg[T_]:=((1-fA[T])/(fB[T]-fA[T]));
Plot[{xg[T],xc[T],1},{T,TA,TB},AxesLabel->{HoldForm[T[K]],HoldForm[x]},PlotLabel->None,LabelStyle->{FontFamily->"Arial",13,GrayLevel[0]},PlotLegends->"Expressions"]