Apparently the answer is -2. how do they get that?

Add the fractions and simplify 1/2+ 1/3

Which of the following is missing in the explicit formula for the compound interest geometric sequence below?

You just purchased two coins at a price of $670 each. because one of the coins is more collectible, you believe that its value will increase at a rate of 7.1 percent per year, while you believe the second coin will only increase at 6.5 percent per year. if you are correct, how much more will the first coin be worth in 15 years?