Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrr} & -7x^{10} & 2x^6 & -3x^3 \\ \color{blue}{ + } & 2x^{10} & -x^6 & x^3 \\ \hline \end{array} $$Step 2: Combine like terms (add each column):
$$ \begin{array}{lrrr} & -7x^{10} & 2x^6 & -3x^3 \\ \color{blue}{ + } & 2x^{10} & -x^6 & x^3 \\ \hline & -5x^{10} & x^6 & -2x^3 \end{array} $$