Step 1: First we have to write polynomials in descending order.
$$ \begin{aligned} P(x) &= 5x^2+x-10 \\ Q(x) &= 3x^2-x-12 \\ \end{aligned} $$Step 2: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrr} & 5x^2 & x & -10 \\ \color{blue}{ + } & 3x^2 & -x & -12 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrr} & 5x^2 & x & -10 \\ \color{blue}{ + } & 3x^2 & -x & -12 \\ \hline & 8x^2 & & -22 \end{array} $$