Answer:
a) p(x)=-0.0006x +21.6
b) $10.80
Explanation:
a) Assuming that p(x) is linear, the slope can be found by:

Applying the point (21000; $9) to the general linear equation gives us the demand function:

b) Revenue is given by the number of tickets sold multiplied by the price, the revenue function is:

The value of x for which the revenue function's derivate is zero is the number of spectators that yield the maximum revenue:

At x = 18,000, tickets price are:
