How do you get the equation for this problem

Hey I have a math test tomorrow and this one review question confused me:

A girl makes and sells shell necklaces. The material for each necklace costs her $4. She has been selling them for $8 each and averaging sales of 40 a week. She found she would lose 4 sales a week by increasing her price by .50 cents. What selling price should be set to maximize profit?

I only figured ou part of the equation, and would appreciate it if you could help me understand how to add the expense cost to it.

(40-4x) (8 + .50x)

Thanks, 10 points best answer

Let x = the # of necklaces she sells.
Let y = her profit (in $)

Let's start with the first equation.
The material starts with $4 costs per necklace.
Therefore, it is y = -4x.
She sells each for $8 per necklace, so you add 8x.
y = -4x + 8x = 4x
Therefore, y = 4x.
She sells 40 necklaces per week, so x = 40 for the first equation.
y = 4 (40) = $160

Next, the second equation.
She increases the price by $0.50, so you add 0.50x.
y = 4x + 0.50x = 4.5x
Therefore, y = 4.5x.
She loses 4 sales per week by increasing the price, so x turns from 40 to 36.
y = 4.5 (36) = $162

$162 is more than $160, so she should increase the price by $0.50 to maximize profits.