Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrrr} & 8x^4 & & -2x^2 & x & -4 \\ \color{red}{ - } & & 3x^3 & -12x^2 & 8x & \\ \hline \end{array} $$Step 2: Change ALL the signs in the second row.
$$ \begin{array}{lrrrrr} & 8x^4 & & -2x^2 & x & -4 \\ \color{red}{ - } & & -3x^3 & 12x^2 & -8x & \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrrrr} & 8x^4 & & -2x^2 & x & -4 \\ \color{red}{ - } & & -3x^3 & 12x^2 & -8x & \\ \hline & 8x^4 & 3x^3 & -14x^2 & 9x & -4 \end{array} $$