576=.75x ($576 is 75% of what?) Divide both sides by .75 to isolate x cancels out the .75 on the side with the x x=576/.75=768 cost of the computer without sales tax To find it with sales tax: First you multiply the cost by the tax rate to get the tax 768*.06=46.08 is the sales tax Then you add the tax to the cost to get the total cost 768+46.08=814.08 is the price of the computer with sales tax
So your two answers are $768 without tax and $814.08 with tax.