Don't define wn as symbolic
vo=input('vo=');
xo=input('xo=');
wn=input('wn=');
syms t;
%syms wn; comment this line
a1=xo;
a2=vo+(wn*xo);
x=(a1+a2*t)*exp(-wn*t);
t=0:0.5:1;
nx=double( subs(x,'t',t));
figure(1);
plot(t,nx);
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