Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrr} & 5x & -9 \\ \color{blue}{ + } & 6x & 4 \\ \hline \end{array} $$Step 2: Combine like terms (add each column):
$$ \begin{array}{lrr} & 5x & -9 \\ \color{blue}{ + } & 6x & 4 \\ \hline & 11x & -5 \end{array} $$