Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrr} & x^4 & 8x^3 & 9x^2 \\ \color{red}{ - } & 5x^4 & 4x^3 & 2x^2 \\ \hline \end{array} $$Step 2: Change ALL the signs in the second row.
$$ \begin{array}{lrrr} & x^4 & 8x^3 & 9x^2 \\ \color{red}{ - } & -5x^4 & -4x^3 & -2x^2 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrr} & x^4 & 8x^3 & 9x^2 \\ \color{red}{ - } & -5x^4 & -4x^3 & -2x^2 \\ \hline & 6x^4 & 12x^3 & 11x^2 \end{array} $$