Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrr} & 2x^2 & 53x & 75 \\ \color{red}{ - } & x^2 & 5x & 5 \\ \hline \end{array} $$Step 2: Change ALL the signs in the second row.
$$ \begin{array}{lrrr} & 2x^2 & 53x & 75 \\ \color{red}{ - } & -x^2 & -5x & -5 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrr} & 2x^2 & 53x & 75 \\ \color{red}{ - } & -x^2 & -5x & -5 \\ \hline & 3x^2 & 58x & 80 \end{array} $$