graph looks like this.
from the graph, you can see.....
when x = 0, the revenue is 12 * 400 = 4800.
when x = 8, the revenue is (12 + 4 = 16) * (400 - 80 = 320) = 5120.
that's the maximum revenue, based on the equation.
when x = 20, the revenue is (12 + 10 = 22) * (400 - 200 = 200) = 4400.
the y-intercept is the value of y when x = 0.
that's 12 * 400 = 4800 income.
the x-intercept is the value of x when y = 0.
that occurs when x = 40.
the price is 12 + 20 = 31.
the number of customers is 400 - 400 = 0.
the revenue is 31 * 0 = 0.
when x = 40, there are no customers left to buy a pizza.
the value of x and the revenue are shown on the graph.
x = the number of increments of .5 added to the price and the number of increments of 10 subtracted from the quantity.
y is the total revnue.
if you want to know what the price is at these points, you have to determine it from the eqution.
when x = 8, price = 12 + .5*8 = 12 + 4 = 16
when x = 8, quantity = 400 - 10*8 = 400 - 80 = 320.
this evaluation of the equation is done for each increment of x that you want to see.
the graph does give you a visual of the shape of the revenue runction and when the maximum revenue is attained.