How to convert profit-demand function to profit-price function

Profit-demand function

P(x) = -(x^2)/20 + 10x - 300

Demand-price function

f(p) = 40 - 20p

Tell me if the question requires more details because I don't actually understand it

x = 40 - 20*p

P(x) = - ((40 - 20*p)^2)/20 + 10*(40 - 20*p) - 300

P(x) = 20 - 120*p - 20*p^2