Explicit formulas for geometric sequences

Solve problems such as: The first four terms in a geometric sequence are 1/3, 1, 3, and 9. Find an explicit formula for the sequence.

Problem

Find an explicit formula for the geometric sequence minus, 1, space, comma, minus, 7, space, comma, minus, 49, space, comma, minus, 343, comma, point, point, point.
d, left parenthesis, n, right parenthesis, equals