$\displaystyle \int_{1}^{3}x^2-x+1~dx$

I have to find this integral using reimann's sums

usually what I do si subtract 3-1/n to get 2/n

and that gives me 2i/n which I then sub into the polynomial and multiply by 2/n to give me the answer(using limits and sums ofcourse)...but for this one it's not working so I'm guessing I shouldn't be using 2i/n???:confused: