Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrr} & & 3w & -1 \\ \color{blue}{ + } & 9w^2 & -3w & 4 \\ \hline \end{array} $$Step 2: Combine like terms (add each column):
$$ \begin{array}{lrrr} & & 3w & -1 \\ \color{blue}{ + } & 9w^2 & -3w & 4 \\ \hline & 9w^2 & & 3 \end{array} $$