Luis wants to buy a skateboard for $79.99. all merchandise is discounted 12%. what is the total cost of the skateboard if luis has to pay a state sales tax of 6.75%?

First you need to figure out the cost of the skateboard after the discount but before the sales tax. By doing 100%-12% you figure out how much of the original cost you are paying 88%. Then you multiply 79.99(original cost) and 88%(how much you pay) which gets you 70.39. Then you figure out the sales tax by multiplying 70.39 by 6.75% which gives you 4.75(the sales tax). And the last thing you need to do is add the cost before tax(70.39) and the tax(4.75). 
The skateboard costs $75.14