Here is how my text book puts it:

Maximizing Revenue

a. $\displaystyle R = f (p) = p . q = p(2400-20p) = 2400p - 20p^2$

b. Manipulate the demand function algebraically by subtracting $\displaystyle q$ from both sides, adding $\displaystyle 20p$ to both sides and dividing both sides by $\displaystyle 20$

The result should be $\displaystyle p = 120-0.05q$

I can't figure out how to get to that result. Any help would be greatly appreciated.