{b1+b2+b3 = 21
{b2+b3+b4 = 42
b2 = b1*q
b3 = b1*q²
b4 = b1*q³
{b1+b1*q+b1*q² = 21
{b1*q+b1*q²+b1*q³ = 42
{b1(1+q+q²) = 21
{b1*q(1+q+q²) = 42
[b1(1+q+q²)] / [b1*q(1+q+q²)] = 21/42
1/q = 21/42
21q = 42
q = 42/21
q = 21.
b1(1+q+q²) = 21
b1(1+2+2²) = 21
b1(3+4) = 21
7b1 = 21
b1 = 21/7
b1 = 3.