The semicolon in the legend call is wrong (in this context).
It would be better to write all that as a script rather than in the Command Window.
When you do that, it works —
x = -5:0.1:5;
s = sin(x);
c = cos(x);
plot(x,s, x,c)
xlabel('x')
ylabel('sine or cosine')
legend('sine','cosine')
EDIT — Corrected typographical errors.
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