Let A be attendance, T be ticket price and R be revenue.
Your Revenue, R = AT which is what you want to maximize.
You are given two points and that the relationship is linear. There are a variety of ways to find the line where A is the dependent variable and T is the independent variable:
Slope is constant for all points on line (A-28K)/(T-9) = (24K-28K)/(12-9) (K just means 1000)
or A - 28K = (-4K/3)(T-9) and A = (-4K/3)T + 40K
The function you want to maximize is R = AT =((-4K/3)T + 40K)T
Usually, one would use calculus to find the T for max R, but the key words indicate that you are in pre-calculus. Since this is a quadratic that is curved downwards, the vertex is the maximum value. You can complete the square to put this in vertex notation:
Let's distribute and multiply in order to make the square term -T2
R = -4K/3*(T2 - 30T)
In order to complete the square we need to add (30/2)2 inside the parentheses and subtract what we added in order to not change the equation:
R = -4K/3*(T2 - 30T + 225) + 4K*225/3
R = -4K/3*(T - 15)2 + 4K*225/3 (The vertex is at T = 15) and the R(T=15) = (4K/3)152 = 300K which you get from plugging T=15 into the original equation or just taking the far right term in this equation.
Hope that helped. Take care.