Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrr} & 5x^2 & 3x & -8 \\ \color{blue}{ + } & 2x^2 & -9x & -7 \\ \hline \end{array} $$Step 2: Combine like terms (add each column):
$$ \begin{array}{lrrr} & 5x^2 & 3x & -8 \\ \color{blue}{ + } & 2x^2 & -9x & -7 \\ \hline & 7x^2 & -6x & -15 \end{array} $$