Step 1: First we have to write polynomials in descending order.
$$ \begin{aligned} P(x) &= -18x^2+x-32 \\ Q(x) &= -x^2+40 \\ \end{aligned} $$Step 2: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrr} & -18x^2 & x & -32 \\ \color{red}{ - } & -x^2 & & 40 \\ \hline \end{array} $$Step 3: Change ALL the signs in the second row.
$$ \begin{array}{lrrr} & -18x^2 & x & -32 \\ \color{red}{ - } & x^2 & & -40 \\ \hline \end{array} $$Step 4: Combine like terms (add each column):
$$ \begin{array}{lrrr} & -18x^2 & x & -32 \\ \color{red}{ - } & x^2 & & -40 \\ \hline & -19x^2 & x & 8 \end{array} $$