Profit=Revenue-Cost, this is something that you just are supposed to know, even though it might not technically be a "math" topic

So for this problem profit

Now can you find the maximum? You will not set profit equal to 0 and solve with the quadratic formula because that will tell you when profit=0 and not the max profit. Instead you should know a rule about the max/min of a parabola in terms of the coefficients of a quadratic equation

Do you know this rule?