Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrrr} & 7x^4 & -5x^3 & 8x^2 & 9x & -13 \\ \color{red}{ - } & 2x^4 & 6x^3 & -4x^2 & -x & 4 \\ \hline \end{array} $$Step 2: Change ALL the signs in the second row.
$$ \begin{array}{lrrrrr} & 7x^4 & -5x^3 & 8x^2 & 9x & -13 \\ \color{red}{ - } & -2x^4 & -6x^3 & 4x^2 & x & -4 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrrrr} & 7x^4 & -5x^3 & 8x^2 & 9x & -13 \\ \color{red}{ - } & -2x^4 & -6x^3 & 4x^2 & x & -4 \\ \hline & 9x^4 & x^3 & 4x^2 & 8x & -9 \end{array} $$