Step 1: First we have to write polynomials in descending order.
$$ \begin{aligned} P(x) &= x^2-8 \\ Q(x) &= 3x^2-4x-10 \\ \end{aligned} $$Step 2: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrr} & x^2 & & -8 \\ \color{blue}{ + } & 3x^2 & -4x & -10 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrr} & x^2 & & -8 \\ \color{blue}{ + } & 3x^2 & -4x & -10 \\ \hline & 4x^2 & -4x & -18 \end{array} $$