You get this error on the line:
f = [(1/k)*300*exp(-(( xj - (L/2)).^2))]';
because you're trying to perform the division 1/k, but k is a function handle.
You defined k as:
k = @(xj) (5 + (1/6)*xj.^2);
This says 'take an input xj and then use that input in the equation'
Maybe you wanted:
f = [(1./k(xj))*300.*exp(-(( xj - (L/2)).^2))]';
I'd also suggest that you consider changing the definition of k to use a different variable name to make more clearly differentiate k's local variable and the outer xj.
I'm also not sure what you intended for the next two lines (which look like scalars so maybe should take in xj(1) and xj(N-1)?
