Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrr} & 4x^2 & -10x & \\ \color{blue}{ + } & 2x^2 & 5x & -5 \\ \hline \end{array} $$Step 2: Combine like terms (add each column):
$$ \begin{array}{lrrr} & 4x^2 & -10x & \\ \color{blue}{ + } & 2x^2 & 5x & -5 \\ \hline & 6x^2 & -5x & -5 \end{array} $$