Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrr} & x & -4 \\ \color{red}{ - } & 16x & -8 \\ \hline \end{array} $$Step 2: Change ALL the signs in the second row.
$$ \begin{array}{lrr} & x & -4 \\ \color{red}{ - } & -16x & 8 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrr} & x & -4 \\ \color{red}{ - } & -16x & 8 \\ \hline & 17x & -12 \end{array} $$