Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrr} & 7x^9 & 9x^5 & 15 \\ \color{red}{ - } & 10x^9 & 8x^5 & 12 \\ \hline \end{array} $$Step 2: Change ALL the signs in the second row.
$$ \begin{array}{lrrr} & 7x^9 & 9x^5 & 15 \\ \color{red}{ - } & -10x^9 & -8x^5 & -12 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrr} & 7x^9 & 9x^5 & 15 \\ \color{red}{ - } & -10x^9 & -8x^5 & -12 \\ \hline & 17x^9 & 17x^5 & 27 \end{array} $$