Answer
$q(20)\approx 233$ million
Work Step by Step
Graphically:
Using the graphing device as in part (a), graph the line $x=20$ in the same window, and find the intersection point with the graph of the demand function.
Numerically:
$q(20)=0.0009\times 20\wedge 2-0.63\times 20+245\approx 233$ million
