Find an explicit formula for the geometric sequence βˆ’ 9   , βˆ’ 18   , βˆ’ 36   , βˆ’ 72 , . . . βˆ’9,βˆ’18,βˆ’36,βˆ’72,...minus, 9, comma, minus, 18, comma, minus, 36, comma, minus, 72, comma, point, point, point. c ( n ) = c(n)=c, left parenthesis, n, right parenthesis, equals