Step 1: First we have to write polynomials in descending order.
$$ \begin{aligned} P(x) &= -x^2+5 \\ Q(x) &= 3x^2-3x-9 \\ \end{aligned} $$Step 2: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrr} & -x^2 & & 5 \\ \color{blue}{ + } & 3x^2 & -3x & -9 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrr} & -x^2 & & 5 \\ \color{blue}{ + } & 3x^2 & -3x & -9 \\ \hline & 2x^2 & -3x & -4 \end{array} $$