Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrrr} & -2x^6 & x^5 & -3x^2 & -4x & 7 \\ \color{red}{ - } & & x^5 & 2x^2 & -4x & 4 \\ \hline \end{array} $$Step 2: Change ALL the signs in the second row.
$$ \begin{array}{lrrrrr} & -2x^6 & x^5 & -3x^2 & -4x & 7 \\ \color{red}{ - } & & -x^5 & -2x^2 & 4x & -4 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrrrr} & -2x^6 & x^5 & -3x^2 & -4x & 7 \\ \color{red}{ - } & & -x^5 & -2x^2 & 4x & -4 \\ \hline & -2x^6 & 2x^5 & -x^2 & -8x & 11 \end{array} $$