m = 1.50;
k = 10.0;
ω0 = Sqrt[k/m];
γ = 0.010*ω0;
F0 = 3.0;
ω = 0.950*ω0;
T = 2.0*(2.0/γ);

A = ((F0/m)/(Sqrt[(((((ω0)^2) - (ω^2)))^2) + (((γ ω))^2)]))

δ = If[
ω0 - ω > 0,
ArcTan[((γ ω)/(((ω0)^2) - (ω^2)))],
If[ω0 - ω < 0,
ArcTan[((γ ω)/(((ω0)^2) - (ω^2)))] + π,
(π/2)]
]

ω1 = (Sqrt[((ω0)^2) - ((γ^2)/4)])

B = (A/(ω1)) (Sqrt[(ω^2) ((Sin[δ])^2) + ((ω0)^2) ((Cos[δ])^2) + γ ω Sin[δ] Cos[δ]])

β = If[
ω0 - ω < 0,
ArcTan[(γ/(2 ω1)) ((((ω^2) + ((ω0)^2))/((ω^2) - ((ω0)^2))))],
If[ω0 - ω > 0,
ArcTan[(γ/(2 ω1)) ((((ω^2) + ((ω0)^2))/((ω^2) - ((ω0)^2))))] + π,
(π/2)]
]

x[t_] := B (E^(-γ t / 2)) Cos[ω1 t + β] + A Cos[ω t - δ]

Plot[x[t], {t,0,T},
AxesLabel -> {"t","x"}]