help with formula

I'm trying to center an image on my screen based on the images width. I want there to be an equal amount pixels between each side of the image. I have the width of the image, and the width of the window is 800 pixels.

I've never been good at math please help.

left: (screen width - image width) / 2

You'll want to round that solution.  And crack open a basic algebra book.

get

to

Huh?