Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrrrr} & 7x^5 & & 4x^3 & -2x^2 & 12x & 5 \\ \color{red}{ - } & & 6x^4 & -9x^3 & x^2 & & -3 \\ \hline \end{array} $$Step 2: Change ALL the signs in the second row.
$$ \begin{array}{lrrrrrr} & 7x^5 & & 4x^3 & -2x^2 & 12x & 5 \\ \color{red}{ - } & & -6x^4 & 9x^3 & -x^2 & & 3 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrrrrr} & 7x^5 & & 4x^3 & -2x^2 & 12x & 5 \\ \color{red}{ - } & & -6x^4 & 9x^3 & -x^2 & & 3 \\ \hline & 7x^5 & 6x^4 & -5x^3 & -x^2 & 12x & 2 \end{array} $$