a) find the price of a hamburger that will maximize the nightly hamburger revenue.

I know that in order to do that, we have to use the point slope formula to find the demand function. I got p-4=[(4-4.4)/(10,000-8000)] [(x-10000).

I don't get why the answer key has p-2=[(2-2.4)/(10,000-8000)] [(x-10000) and still got the correct answer?