Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrr} & 5y^2 & -6y & -11 \\ \color{red}{ - } & 6y^2 & 2y & 5 \\ \hline \end{array} $$Step 2: Change ALL the signs in the second row.
$$ \begin{array}{lrrr} & 5y^2 & -6y & -11 \\ \color{red}{ - } & -6y^2 & -2y & -5 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrr} & 5y^2 & -6y & -11 \\ \color{red}{ - } & -6y^2 & -2y & -5 \\ \hline & 11y^2 & -4y & -6 \end{array} $$