Where did you use the derivative?

x = Price

Demand(x) = a + bx -- We're assuming a linear model.

Revenue(x) = x*Demand(x) = ax + bx^2

Now we have clues:

Demand(4) = a + b(4) = 10000

Demand(4.4) = a + b(4.4)= 8000

I get a = 30000 and b = -5000

Revenue(x) = 30000*x - 5000*x^2

dRevenue/dx = 30000 - 10000*x

Find zero: 30000 - 10000*x ==> x = 3

Revenue(3) = 45000

Write down definitions. Write clearly and completely. Make sure at least YOU can follow your work.