Price * Number Sold = Total Income.
x(y) = Price based on number sold = 400 - 0.3y
y = Number sold
y*x(y) = Total Income = z(y) = y*(400-0.3y) = 400y - 0.3y^2
What do you know about parabolas with negative leading coefficients?
Since you are to use Differential Calculus,
Where is that zero?
-0.598y + 394 = 0 and Solve for y.
You are beginning to worry me. It seems as though you are a little too eager to give up on a problem if it does not immediately make sense. Maybe it would help to know why you are in this class. Have you had success in the prerequisite mathematics courses?