Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrr} & 8x^3 & -10x^2 & 28x & -16 \\ \color{red}{ - } & & & x & -3 \\ \hline \end{array} $$Step 2: Change ALL the signs in the second row.
$$ \begin{array}{lrrrr} & 8x^3 & -10x^2 & 28x & -16 \\ \color{red}{ - } & & & -x & 3 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrrr} & 8x^3 & -10x^2 & 28x & -16 \\ \color{red}{ - } & & & -x & 3 \\ \hline & 8x^3 & -10x^2 & 29x & -19 \end{array} $$