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