Step 1: First we have to write polynomials in descending order.

$$ \begin{aligned} P(x) &= 2n^4+13n-9 \\ Q(x) &= 7n^4+n+12 \\ \end{aligned} $$

Step 2: Write second polynomial below the first one. Making sure like terms are lined up.

$$ \begin{array}{lrrr} & 2n^4 & 13n & -9 \\ \color{red}{ - } & 7n^4 & n & 12 \\ \hline \end{array} $$

Step 3: Change ALL the signs in the second row.

$$ \begin{array}{lrrr} & 2n^4 & 13n & -9 \\ \color{red}{ - } & -7n^4 & -n & -12 \\ \hline \end{array} $$

Step 4: Combine like terms (add each column):

$$ \begin{array}{lrrr} & 2n^4 & 13n & -9 \\ \color{red}{ - } & -7n^4 & -n & -12 \\ \hline & 9n^4 & 14n & 3 \end{array} $$

Polynomial Operations Calculator