The ‘1/13s’ I believe should be the step in your ‘t’ vector.
This is how I interpret the problem:
tlim = 1;
tc = linspace(0, tlim, 250);
t = [0:(1/13):tlim];
y = @(t) sin(8*pi*t);
g = @(t) sin(18*pi*t);
That’s an impressive illustration of the phenomenon the problem is asking you to observe. (I made ‘y’ and ‘g’ anonymous functions for convenience.)