Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrr} & 8x^3 & -10x^2 & 30 \\ \color{red}{ - } & 4x^3 & -7x^2 & -6 \\ \hline \end{array} $$Step 2: Change ALL the signs in the second row.
$$ \begin{array}{lrrr} & 8x^3 & -10x^2 & 30 \\ \color{red}{ - } & -4x^3 & 7x^2 & 6 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrr} & 8x^3 & -10x^2 & 30 \\ \color{red}{ - } & -4x^3 & 7x^2 & 6 \\ \hline & 12x^3 & -17x^2 & 24 \end{array} $$