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