The only problems were in not using element-wise operations (use the ‘dot operator’ with the exponentiation an d multiplication operators), and presenting the correct arguments to plot. With those corrections (the only ones I made to the posted code), the original code works. See Array vs. Matrix Operations to understand about element-wise operations.
n = -5:1:5;
x = ((1 / 7) .^ -n) .* heaviside(-n) + ((2 / 3) .^ n) .* heaviside(n) + ((3/5) .^ n) .* heaviside(n);
figure(1);
plot(n, x, '-b')
figure(2);
stem(n, x, '-b', 'filled')
That sometimes makes more sense when plotting discrete values.
..
