Assume that the demand function for a certain commodity has the form
$$ p=\sqrt{-a x^{2}+b} \quad(a \geq 0, b \geq 0) $$
where $x$ is the quantity demanded, measured in units of a thousand and $p$ is the unit price in dollars. Suppose the quantity demanded is $6000(x=6)$ when the unit price is $\$ 8$ and $8000(x=8)$ when the unit price is $\$ 6 .$ Determine the demand equation. What is the quantity demanded when the unit price is set at $\$ 7.50 ?$