Step 1: First we have to write polynomials in descending order.
$$ \begin{aligned} P(x) &= 2x^3-2x-9 \\ Q(x) &= x^3+6x^2+11 \\ \end{aligned} $$Step 2: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrr} & 2x^3 & & -2x & -9 \\ \color{blue}{ + } & x^3 & 6x^2 & & 11 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrrr} & 2x^3 & & -2x & -9 \\ \color{blue}{ + } & x^3 & 6x^2 & & 11 \\ \hline & 3x^3 & 6x^2 & -2x & 2 \end{array} $$