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