To find the sum of the first n terms of a geometric sequence use the formula

Sn = a1(1 - r^n)/(1 - r)

where n is the number of terms, a1 is the first term and r is the common ratio.

In our case n = 18, a1 = 3, r = 2

S18 = 3(1 - 2^18)/(1 - 2)

S18 = 786,429

the sum of the first 18 terms of the gp is 786,429.