Step 1: First we have to write polynomials in descending order.
$$ \begin{aligned} P(x) &= 3x^5+7x^2-4x+8 \\ Q(x) &= 4x^4+13x+7 \\ \end{aligned} $$Step 2: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrrr} & 3x^5 & & 7x^2 & -4x & 8 \\ \color{red}{ - } & & 4x^4 & & 13x & 7 \\ \hline \end{array} $$Step 3: Change ALL the signs in the second row.
$$ \begin{array}{lrrrrr} & 3x^5 & & 7x^2 & -4x & 8 \\ \color{red}{ - } & & -4x^4 & & -13x & -7 \\ \hline \end{array} $$Step 4: Combine like terms (add each column):
$$ \begin{array}{lrrrrr} & 3x^5 & & 7x^2 & -4x & 8 \\ \color{red}{ - } & & -4x^4 & & -13x & -7 \\ \hline & 3x^5 & 4x^4 & 7x^2 & 9x & 15 \end{array} $$