Can anyone sohw me how to do Riemann Integrals Sums on matlab.

The question I have to do is this:

Using Matlab, calculate an approximation for the definite integral of

g(x)

= 2 − x^3 on the interval [0,4] using 100 equal subintervals.

What I do is use the command

rsums(2-x^3,0,4)

Then I move the bottom to 100 (so that there are 100 subintervals) And on the top it gives me -13.999200

Is this right? It doesnt seem right. Shouldnt the approximation be closer to -56 (which is what the exact signed area is)? And is there any other way to do it?