Hmm they're both even numbers so maybe we can start by cutting each number in half.

18 and 48 had 2 as a common factor.
So factoring a 2 out of each number was the same as cutting each number in half. Try to do something similar with the 9 and 24. They each have something in common.
OptionC is the answer..!!
Hope this helps you!!!
X is 9 Y is 3 and Z is 12
I believe the number of terms in the sequence would be represented by the value on top of the sigma sign, which is 8. There would be 8 terms.