If xnx_nxn is a geometric sequence with common ratio rrr, and ∑n=13xn=21\sum_{n=1}^3 x_n = 21∑n=13xn=21 and ∑n=13xn3=189×9\sum_{n=1}^3 x_n^3 = 189 \times 9∑n=13xn3=189×9, what is rrr?
222
333
444
1/21/21/2