Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrr} & 5x^3 & 3x^2 & & 5 \\ \color{red}{ - } & 7x^3 & -9x^2 & 8x & -5 \\ \hline \end{array} $$Step 2: Change ALL the signs in the second row.
$$ \begin{array}{lrrrr} & 5x^3 & 3x^2 & & 5 \\ \color{red}{ - } & -7x^3 & 9x^2 & -8x & 5 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrrr} & 5x^3 & 3x^2 & & 5 \\ \color{red}{ - } & -7x^3 & 9x^2 & -8x & 5 \\ \hline & 12x^3 & -6x^2 & 8x & \end{array} $$