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