What is the sale price of $7.50 20% off 5.75% tax

Assuming that you mean 20% off of $7.50 and then taking 5.75% tax of that, then you would:
- Take 20% of $7.50 (turn 20% into a decimal and multiply it by 7.50)
.2 * 7.50 = 1.5
This 1.5 is the amount that is being taken off, because it is 20% off. So to get 20% off you would subtract 1.5 from 7.5 to get $6 total. 
After you get your $6 total, you need to:
- Take 5.75% of $6 (turn 5.75% into a decimal and multiply it by 6)
.0575 * 6 = .345
This .345 is being added, but remember that we're dealing with money which should really only go to two decimal places, so it's safe to round up and say .35 instead of .345. 
From this, add .35 to 6 and that will get you your final dollar amount. 
6 + .35 = $6.35
Remember that we add the tax and not subtract it because tax is something extra that we pay on goods.