"R(x) is should be price * units sold"
Let's go with that.
Price = p
Units Sold = Demand = q = -40p + 600
There is a bit of a problem with this, as Units Sold is not ALWAYS the same as Demand. If you have a supply problem, it doesn't work. However, in an environment of unlimited resources and availability, we have:
Revenue = Price * Units Sold = p * q = p * (-40p + 600) = -40p^2 + 600p