b2 + b3 = 6
b4 = 13.5
b2 = b1*q
b3 = b1*q^2
b4 = b1*q^3
b2 + b3 = b1*q + b1*q^2 = b1*(q + q^2) = 6
b4 = b1*q^3 = 13.5 → b1 = 13.5/q^3
13.5/q^3*(q + q^2) = 6 → q = - 3/4 ∨ q = 3
b1*((- 3/4) + (- 3/4)^2) = 6 → b1 = -32 → bn = -32*(-3/4)^(n - 1)
b1*((3) + (3)^2) = 6 → b = 0.5 → bn = 0.5*3^(n - 1)