Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrrrrr} & s^6 & 7s^5 & 2s^4 & 9s^3 & 10s^2 & 12s & 15 \\ \color{red}{ - } & s^6 & 9s^5 & 8s^4 & 9s^3 & 12s^2 & 15s & 20 \\ \hline \end{array} $$Step 2: Change ALL the signs in the second row.
$$ \begin{array}{lrrrrrrr} & s^6 & 7s^5 & 2s^4 & 9s^3 & 10s^2 & 12s & 15 \\ \color{red}{ - } & -s^6 & -9s^5 & -8s^4 & -9s^3 & -12s^2 & -15s & -20 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrrrrrr} & s^6 & 7s^5 & 2s^4 & 9s^3 & 10s^2 & 12s & 15 \\ \color{red}{ - } & -s^6 & -9s^5 & -8s^4 & -9s^3 & -12s^2 & -15s & -20 \\ \hline & 2s^6 & 16s^5 & 10s^4 & 18s^3 & 22s^2 & 27s & 35 \end{array} $$