Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrr} & 6c^5 & -2c^4 & & -2 \\ \color{blue}{ + } & & & -11c^2 & -15 \\ \hline \end{array} $$Step 2: Combine like terms (add each column):
$$ \begin{array}{lrrrr} & 6c^5 & -2c^4 & & -2 \\ \color{blue}{ + } & & & -11c^2 & -15 \\ \hline & 6c^5 & -2c^4 & -11c^2 & -17 \end{array} $$