Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrrr} & 10x^4 & -4x^3 & -7x^2 & 5x & 9 \\ \color{red}{ - } & 2x^4 & -5x^3 & -4x^2 & 9x & 3 \\ \hline \end{array} $$Step 2: Change ALL the signs in the second row.
$$ \begin{array}{lrrrrr} & 10x^4 & -4x^3 & -7x^2 & 5x & 9 \\ \color{red}{ - } & -2x^4 & 5x^3 & 4x^2 & -9x & -3 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrrrr} & 10x^4 & -4x^3 & -7x^2 & 5x & 9 \\ \color{red}{ - } & -2x^4 & 5x^3 & 4x^2 & -9x & -3 \\ \hline & 12x^4 & -9x^3 & -11x^2 & 14x & 12 \end{array} $$