Geometric sequences go up due to a common ratio. Here the common ratio can be worked out by dividing a term by its previous term e.g term 2 divided by term 1.

Therefore the common ratio is 6.
Answer:
Step-by-step explanation:
You need to set up a proportion.
(21 - 15) / 21 = 4/(x + 4)
6 / 21 = 4/(x + 4)
2/7 = 4/(x + 4)
2(x + 4) = 7*4
2x + 8 = 28
2x + 8-8= 28 - 8
2x = 20
x = 20/2
x = 10
6 because there would be the same number of tulips in each vase and same name of vases. 6 x 6 is 36