Explanation:
a geometric sequence means that every term is created by multiplying the previous term by a certain constant factor. this factor is called the "common ratio".
so,
6×r = 18
r = 18/6 = 3
and a quick check tells us this works also for the next terms (18×3 = 54, 54×3 = 162), so it is indeed a geometric sequence with common ratio 3.
a0 = 6
a1 = a0 × 3 = 6×3 = 18
a2 = a1 × 3 = a0 × 3×3 = 6×3² = 54
an = a0 × 3^n = 6 × 3^n
so,
f(x) = 6×3^x, x is integer, x >=0