Step 1: First we have to write polynomials in descending order.
$$ \begin{aligned} P(x) &= -2x^3-x^2+3 \\ Q(x) &= 3x^3+7x^2+6 \\ \end{aligned} $$Step 2: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrr} & -2x^3 & -x^2 & 3 \\ \color{blue}{ + } & 3x^3 & 7x^2 & 6 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrr} & -2x^3 & -x^2 & 3 \\ \color{blue}{ + } & 3x^3 & 7x^2 & 6 \\ \hline & x^3 & 6x^2 & 9 \end{array} $$