Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrr} & 2x^3 & 4x^2 & -5x & -3 \\ \color{red}{ - } & -x^3 & 5x^2 & & -4 \\ \hline \end{array} $$Step 2: Change ALL the signs in the second row.
$$ \begin{array}{lrrrr} & 2x^3 & 4x^2 & -5x & -3 \\ \color{red}{ - } & x^3 & -5x^2 & & 4 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrrr} & 2x^3 & 4x^2 & -5x & -3 \\ \color{red}{ - } & x^3 & -5x^2 & & 4 \\ \hline & x^3 & 9x^2 & -5x & -7 \end{array} $$