`3, 12, 21, 30...` Determine whether the sequence is geometric. If so, find the common ratio.

You need to verify if the sequence is a geometric sequence, hence, you need to use the following property, such that:

`a_n = sqrt(a_(n-1)*a_(n+1))`

Replacing 12 for `a_n` , 3 for `a_(n-1)` and 21 for `a_(n+1)` yields:

`12 = sqrt(3*21) => 12 = sqrt(3^2*7) => 12 != 3sqrt7`

Since the given sequence is not geometric, the ratio cannot be found.