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