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2^x + 2^(x+1) + 2^(x+2) + 2^(x+3) = 15/2
2^x + (2^x)2 + (2^x)4 + (2^x)8 = 15(2^-1)
Factor out 2^x
(2^x ) (1+2+4+8) = 15(2^-1)
(2^x) (15) = 15(2^-1)
2^x = 2^-1
x=-1