Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrr} & 7x^2 & 8x & -5 \\ \color{blue}{ + } & 3x^2 & -4x & -7 \\ \hline \end{array} $$Step 2: Combine like terms (add each column):
$$ \begin{array}{lrrr} & 7x^2 & 8x & -5 \\ \color{blue}{ + } & 3x^2 & -4x & -7 \\ \hline & 10x^2 & 4x & -12 \end{array} $$