Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrrr} & 3x^5 & & 7x^2 & -4x & 8 \\ \color{red}{ - } & & 4x^4 & 8x^2 & -3x & 7 \\ \hline \end{array} $$Step 2: Change ALL the signs in the second row.
$$ \begin{array}{lrrrrr} & 3x^5 & & 7x^2 & -4x & 8 \\ \color{red}{ - } & & -4x^4 & -8x^2 & 3x & -7 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrrrr} & 3x^5 & & 7x^2 & -4x & 8 \\ \color{red}{ - } & & -4x^4 & -8x^2 & 3x & -7 \\ \hline & 3x^5 & 4x^4 & 15x^2 & -7x & 15 \end{array} $$