Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrr} & 4g^3 & 6g^2 & 3g & 9 \\ \color{red}{ - } & 3g^3 & 5g^2 & g & 1 \\ \hline \end{array} $$Step 2: Change ALL the signs in the second row.
$$ \begin{array}{lrrrr} & 4g^3 & 6g^2 & 3g & 9 \\ \color{red}{ - } & -3g^3 & -5g^2 & -g & -1 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrrr} & 4g^3 & 6g^2 & 3g & 9 \\ \color{red}{ - } & -3g^3 & -5g^2 & -g & -1 \\ \hline & 7g^3 & 11g^2 & 4g & 10 \end{array} $$