Why do you need to find the common denominator to simplify

For the following geometric sequence, find the recursive formula. {-80, 20, -5, } i need : ')

Urgent question in mathematics. solve this to get 15 points[tex] {2}^{100} - {2}^{99} - {2}^{98} {2}^{2} - {2}^{1} - {2}^{0}[/tex]a) 0 b) 1 c) 2 d) 2^50

Me on this i beg you on this to answer!