Summations of a summation without upper bound as a lemma

Question

So I have a paper I'm writing for a class that requires me to include a proof. I have the proof all sorted out on paper but I'm having a very hard time figuring out where to begin in Latex. I need to include the picture below as a lemma so it will appear in the equation listed in the code below. I already have \newtheorem{lemm}{Lemma} at the top of my document. Basically, if I can figure out how to code the equation from the image into Latex as a lemma, I think I'll have everything I need to code the rest of the proof. I'm very new at Latex so this is a daunting task for me. Any feedback or suggestions would be much appreciated.

\begin{lemm}  
A(x)B(x)=  
\end{lemm}

Answer 1

Something like this?

\documentclass{article}
\usepackage{amsthm}
\newtheorem{lemm}{Lemma}
\begin{document}
\begin{lemm}
\[
A(x)B(x) = \sum_{k\ge0} \biggl( \sum_{i=0}^{k}a_ib_{k-i} \biggr) x^k
\]
\end{lemm}

\end{document}