The answer is 16(1/2)^n-1.
Now how to get answer without a calculator, I'm afraid I can't tell you an exact answer, but if you are allowed to use them in class, I THOROUGHLY recommend getting a graphing calculator for questions like these. You just have to plug in the equation and see what you get.
For a method without the calculator, take each x (2,3,4,5) and plug them into the equation and see which one will match up. It's a tedious method, but it'll help you get the correct answer.
