You equation returns numbers with both real and imaginary parts. Following code plots both
f = @(t) ((5.14*10^(-4))-(6.44*10^(-5)*1i)).*exp(5*1i*t)+exp(-0.796*t).*(0.99+6.44*10^(-5)*1i).*cos(7.96*t)+exp(-0.796*t).*(-0.099-3.29*10^(-4)*1i).*sin(7.96*t);
t = linspace(0,20,1000);
f_value = f(t);
subplot(2,1,1);
plot(t, real(f_value))
title('real');
subplot(2,1,2);
plot(t, imag(f_value))
title('imaginary');
