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