Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrr} & 3x^3 & 3x^2 & & 9 \\ \color{red}{ - } & 5x^3 & -7x^2 & 6x & -9 \\ \hline \end{array} $$Step 2: Change ALL the signs in the second row.
$$ \begin{array}{lrrrr} & 3x^3 & 3x^2 & & 9 \\ \color{red}{ - } & -5x^3 & 7x^2 & -6x & 9 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrrr} & 3x^3 & 3x^2 & & 9 \\ \color{red}{ - } & -5x^3 & 7x^2 & -6x & 9 \\ \hline & 8x^3 & -4x^2 & 6x & \end{array} $$