Let x = the ticket cost reduction in $. The ticket price function will be:
Price = $9 - x
The demand function (number of spectators) is:
p(x) = 27,000 + 5000x
The revenue is:
R(x) = (Demand)(Price)
R(x) = (27,000 + 5000x)(9 - x) = -5000x2 + 180,000x + 243,000
To find the optimal revenue, take the derivative of R(x) wrt x, set it to zero, and solve for x. Price = $9 - x. Answer is $7.20.