But, I'm not understanding the Lagrange Remainder.

for some

Here's a problem:
"Use the Lagrange form of the remainder to prove that the Maclaurin series converges to the generating function from the given function"

x* e^x

I got the Maclaurin series of that function.

$\displaystyle xe^x= x + x^2 + \frac{x^3}{2!}$

x^(n+1)/n! with the proper sum notation.

What I don't get is how to put this into the equation at the top. I might be able to correctly put a few things in the formula, such as the function, but I don't know how to solve or finish it.