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