How do you subtract sales tax?

I'm kinda assuming you mean you want to find the pre-tax price of something when you know the post-tax price of that item. easiest way to do that is to cross-multiply. let's say your sales tax rate is 7%, then a pre-tax amount of $1 would equal $1.07 after tax. that ratio stays the same, so if your total cost (including tax) for an item is $27.77, you can say:

$1.00 X

------- = ---------

$1.07 $27.77

then just solve for X:

X = (1 * 27.77) / 1.07 = 25.95

just substitute your post tax price for a one dollar item in place of the $1.07, and the post tax price of your item for the $27.77 to find the pre-tax price of your item.