Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrrrrr} & x^6 & 7x^5 & 2x^4 & 9x^3 & 10x^2 & 12x & 15 \\ \color{blue}{ + } & x^6 & 9x^5 & 8x^4 & 9x^3 & 12x^2 & 15x & 20 \\ \hline \end{array} $$Step 2: Combine like terms (add each column):
$$ \begin{array}{lrrrrrrr} & x^6 & 7x^5 & 2x^4 & 9x^3 & 10x^2 & 12x & 15 \\ \color{blue}{ + } & x^6 & 9x^5 & 8x^4 & 9x^3 & 12x^2 & 15x & 20 \\ \hline & 2x^6 & 16x^5 & 10x^4 & 18x^3 & 22x^2 & 27x & 35 \end{array} $$