From the problem description we know that a one dollar decrease in ticket price results in a 4000 person increase in attendance. We can therefore write:
A = Ao - T * 4000
Where A is the attendance, Ao is a constant, and T is the ticket price. From the data given we have
28000 = Ao - 8 * 4000
32000 = Ao - 7 * 4000
or
Ao = 60000.
We also know that revenue R is
R = A * T = (Ao - T * 4000) * T = Ao * T - 4000 * T^2
To find the maximu we take the first derivative of R wrt T and set it to zero
dR/dT = Ao - 8000 * T = 0
So the maximum is at a ticket price T = Ao / 8000 = $7.50