a(n) = 2q^(n-1),q>0.
12 = a(3) - a(2) = 2q^2 - 2q,
0 = q^2 - q - 6 = (q-3)(q+2),
q = 3.
a(n) = 2*3^(n-1).
b(n) = 1 + 2(n-1) = 2n - 1.
s(n) = a(1)+a(2)+...+a(n) + b(1)+b(2)+...+b(n)
= 2[3^n - 1]/(3-1) + n(n+1) - n
= 3^n - 1 + n^2
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