That's probably still not specific enough - what is the discount, and how much tax is there on what amount of money?
Having said that for a discount of d% you can say that given a starting price of x the new price is
x-d*x/100, and similarly you can work what a t% tax rate comes to in real terms by saying that
tax=t*y/100, where y is the amount of money that is being taxed.
If you're taxing on the discount price (what you pay after the discount) then in the second formula you would use
y=x-d*x/100, and either add or subtract this result to y found earlier.
That's just about all I think there is to it.
As an example: if there's a 30% discount with VAT of 17.5% (the rate in the UK from January 2010) then for an item the shop would sell at £10 normally (excluding VAT) would cost the customer [17.5*(10-(30*10/100))/100]+[10-(30*10)/100]=£8.22 if VAT is worked out based on the discount price, or [17.5*10/100]+[10-(30*10)/100]=£8.75 if, as is probably more usual, VAT is worked out based on the normal selling price.
This is rather ugly, I grant you, but that's economics for you.