You did not write the question correctly but I will try to answer the question according to 2 possible interpretations of your question.
Case 1:
Given the geometric sequence:

Common ratio is given by

The explicit rule of a geometric sequence is given by

Therefore, the explicit rule of the given sequence is

Case 2:
Given the geometric sequence:

Common ratio is given by

The explicit rule of a geometric sequence is given by

Therefore,
the explicit rule of the given sequence is