Need Help

To find the minimum, set the 1st derivative to 0
0.04x - 3.4 = 0
Multiply by 100
4x = 340

x = 85
-------
Sub 85 for x in the equation
C(x)= 0.02x^2-3.4x + 150
C(85) = 0.02*85*85 - 3.4*85 + 150
= $583.50
That's the cost of 85 bars.
For one, $583.50/85 = $6.865 

I made the font Pretty!! :)

Here's a graph......if that's what you're asking for......https://www.desmos.com/calculator/naioeft7bu

The vertex is at (85, 5.5)