For my understanding you want to plot the above equation for t=0,1,2,3,...,15
There is no imaginary part for above equation so
As an example code
t = linspace(0,15,16);
x = cos(t/4)+cos(3*t/4);
plot(t, x, 'b-', 'LineWidth', 2);
xlabel('t', 'FontSize', 20);
ylabel('x', 'FontSize', 20);