{tfm,ssm}={(TransferFunctionModel[((E^(-5 s))/(1+2 s)), ]),(StateSpaceModel[0, 1, 0, -3, -(2/3), (SystemsModelDelay[(7/3)]), 1, 0, 0, ])};

series=SystemsModelSeriesConnect[tfm,ssm]//Simplify

Plot[Evaluate[Table[OutputResponse[sys,UnitStep[t],{t,0,20}],{sys,{tfm,ssm,series}}]],{t,0,20},PlotLegends->{"tfm","ssm","series"}]

feedback=SystemsModelFeedbackConnect[ssm,tfm]//Simplify

Plot[Evaluate[Table[OutputResponse[sys,UnitStep[t],{t,0,20}],{sys,{tfm,ssm,feedback}}]],{t,0,20},PlotLegends->{"tfm","ssm","feedback"}]