$150 necklace and a 6.5% sales tax:
First, the easiest way to figure this out is to convert the percentage (6.5) into a number that you can calculate with. Take your percentage and divide it over the number 100.

Now that we have a number that we can calculate with, our next step is to multiply our new number with the total amount that we have to spend on the necklace.

Knowing that you are going to have to pay an extra $9.75 due to sales taxes, you can now add the tax up with the original price to get the total value you are going to have to pay.

Answer:
$159.75