You need to use elementwise multimplication.
let look at this:
A = (3.5.^(-.5*x)); % Here you will get a vector with 601 elements
B = (cos(6*x)); % Here you will get also a vector with 601 elements
You cannot just multiply, You need to multiplay each element in place K of A with element in place K of B.
So just replace:
y = (3.5.^(-.5*x))*(cos(6*x));
with this:
y = (3.5.^(-.5*x)).*(cos(6*x));
