Step 1: First we have to write polynomials in descending order.
$$ \begin{aligned} P(x) &= -6a^3+a^2+7a \\ Q(x) &= -3a^2+a+2 \\ \end{aligned} $$Step 2: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrr} & -6a^3 & a^2 & 7a & \\ \color{blue}{ + } & & -3a^2 & a & 2 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrrr} & -6a^3 & a^2 & 7a & \\ \color{blue}{ + } & & -3a^2 & a & 2 \\ \hline & -6a^3 & -2a^2 & 8a & 2 \end{array} $$