Hi Saitenji,

You don't need a reason to ask for help.

Cost = 800 + 0.04x + 0.0002x²

Cost/Unit = 800/x + 0.04 + 0.0002x

To find a minimum/maximum we usually differentiate and find where the rate of change is zero (I hope you are OK with a little calculus)

derivative of 800/x + 0.04 + 0.0002x = 800 (-1/x²) + 0 + 0.0002

And you want that to be zero, so: -800/x² + 0.0002 = 0

Rearranging: 800/x² = 0.0002

Rearranging: x = √(800/0.0002) = 2000

(This is where the slope of the line is zero, it could be a minimum or maximum, I am guessing it is the minumum!)