I'm using the weighted mean formula to solve this, but I keep getting the answer 12.5 and can't seem to find what I'm doing wrong. Please explain your answer. Thank you!

Solve: 25 points find the fifth term of an increasing geometric progression if the first term is equal to 7−3 √5 and each term (starting with the second) is equal to the difference of the term following it and the term preceding it.