Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrr} & 4x^3 & 3x^2 & -x & 2 \\ \color{red}{ - } & 5x^3 & -3x^2 & x & -4 \\ \hline \end{array} $$Step 2: Change ALL the signs in the second row.
$$ \begin{array}{lrrrr} & 4x^3 & 3x^2 & -x & 2 \\ \color{red}{ - } & -5x^3 & 3x^2 & -x & 4 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrrr} & 4x^3 & 3x^2 & -x & 2 \\ \color{red}{ - } & -5x^3 & 3x^2 & -x & 4 \\ \hline & 9x^3 & & & -2 \end{array} $$