Step 1: First we have to write polynomials in descending order.
$$ \begin{aligned} P(x) &= -3q^4+7q^3+6 \\ Q(x) &= 9q^4+2q^2-4 \\ \end{aligned} $$Step 2: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrr} & -3q^4 & 7q^3 & & 6 \\ \color{blue}{ + } & 9q^4 & & 2q^2 & -4 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrrr} & -3q^4 & 7q^3 & & 6 \\ \color{blue}{ + } & 9q^4 & & 2q^2 & -4 \\ \hline & 6q^4 & 7q^3 & 2q^2 & 2 \end{array} $$