The below is a MMA cell code copied into this post. Marking whats been pasted, then Ctrl+K (Windows). Copy back to a notebook of your own, and try it out.
Manipulate[
(* Preliminaries *)
a = (F*Sin[\[Gamma]])/(2*c*\[Omega]);
\[Sigma] = (3*\[Mu]*a^2)/(8*\[Omega]) - (F*Cos[\[Gamma]])/(2*a*\[Omega]);
data = Transpose@{\[Sigma], a};
(* Output *)
ListLinePlot[data,
PlotRange -> {{-1, 1}, {0, 1}},
AxesLabel -> {sigma, amplitude},
PlotLabel -> "MMS Duffing frequency Response"
],
(* Active controls *)
{{\[Mu], 1, "Nonlienarity"}, 1, 5},
{{F, 0.1, "Force"}, 0.1, 1},
{{c, 0.01, "Damping"}, 0.01, 1},
(* Local variables as controls *)
{\[Omega], 3.15, ControlType -> None},
{{\[Gamma], Range[0.001, 3.1415, 0.001]}, ControlType -> None},
{a, ControlType -> None},
{\[Sigma], ControlType -> None},
{data, ControlType -> None}
]