1) When you have an exponent being raised to an exponent, you multiply them. 2*3 is 6, so you have x^6, which would make C your answer.
2) You reduce them by subtracting the top ones from the bottom ones, so it becomes x^2*z. The y's cancel out, 6-4 is 2 and 9-8 is 1. Your answer is C again.
3) As you did in one, simply multiply out the top first. The top them becomes 9x^12*y^18. Subtract the bottom exponents to get 9x^6y^9/3, which then becomes 3x^6*y^9, which is D.
4) When you have a square root over a square root, you can put the numbers inside like this:
. 81 over 27 is equal to 3, so you have sqrt3 as your answer - A.
5) You have sqrt2*2*sqrt3. You can put 2*3 under the square root together, and they multiply to 6, so you have 2sqrt6 - D.