Completely factor the following expression: $9x^2+3x$.
The largest common factor of $9x^2$ and $3x$ is $3x$. We factor $3x$ out of each term to get\begin{align*}
9x^2+3x &= 3x\cdot 3x + 3x \cdot 1\\
&= 3x(3x+1).
\end{align*}