By the way you called your function:
[f]= NC(-3,0,fun)
you're trying to assign to the -3rd element of f with the code:
f(a)= fun(a); %y value for lower limit
and to the 0th element of f with:
f(b)= fun(b); %y value for upper limit
Neither of those are valid in MATLAB. The first element of an array in MATLAB is element 1.
Try just assigning to fa and fb instead and using those variables later in your code.
