How many terms of the series 2 + 6 + 18 + …… must be taken to make the sum equal to 728?

In a G.P., the ratio between the sum of first three terms and that of the first six terms is 125 : 152. Find its common ratio.

The first term of a G.P. is -3 and the square of the second term is equal to its 4^th term. Find its 7^th term.