Step 1: First we have to write polynomials in descending order.
$$ \begin{aligned} P(x) &= 8n^2-3n+6 \\ Q(x) &= 7n^3-3n^2-1 \\ \end{aligned} $$Step 2: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrr} & & 8n^2 & -3n & 6 \\ \color{red}{ - } & 7n^3 & -3n^2 & & -1 \\ \hline \end{array} $$Step 3: Change ALL the signs in the second row.
$$ \begin{array}{lrrrr} & & 8n^2 & -3n & 6 \\ \color{red}{ - } & -7n^3 & 3n^2 & & 1 \\ \hline \end{array} $$Step 4: Combine like terms (add each column):
$$ \begin{array}{lrrrr} & & 8n^2 & -3n & 6 \\ \color{red}{ - } & -7n^3 & 3n^2 & & 1 \\ \hline & 7n^3 & 5n^2 & -3n & 5 \end{array} $$