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

$$ \begin{aligned} P(x) &= 18p \\ Q(x) &= 3p \\ \end{aligned} $$

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

$$ \begin{array}{lr} & 18p \\ \color{red}{ - } & 3p \\ \hline \end{array} $$

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

$$ \begin{array}{lr} & 18p \\ \color{red}{ - } & -3p \\ \hline \end{array} $$

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

$$ \begin{array}{lr} & 18p \\ \color{red}{ - } & -3p \\ \hline & 21p \end{array} $$

Polynomial Operations Calculator