I think what you have is correct, except in the last command you should be plotting q rather than S and x. Also if you want all the lines to appear on the same figure, you need to plot them together or hold the figure.
figure
hold on
n=1:50;
x=rand(size(n));
plot(x,'r');
S=2*(n.*(0.9).^n);
plot(S,'b--')
q = S+x;
plot(q,'r-.');
legend('x', 'S', 'q');