Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrr} & 5x^3 & 5x^2 & & 5 \\ \color{red}{ - } & 6x^3 & -5x^2 & 18x & -5 \\ \hline \end{array} $$Step 2: Change ALL the signs in the second row.
$$ \begin{array}{lrrrr} & 5x^3 & 5x^2 & & 5 \\ \color{red}{ - } & -6x^3 & 5x^2 & -18x & 5 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrrr} & 5x^3 & 5x^2 & & 5 \\ \color{red}{ - } & -6x^3 & 5x^2 & -18x & 5 \\ \hline & 11x^3 & & 18x & \end{array} $$