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