Answer:
3 : 7.
Explanation:
For every 3 stars, there are 7 circles. If 3 stars is written first, you'll need to write 3 first in your answer, and 7 second.
This is what it would look like: 3 : 7
With ratios, you could also write,
3 to 7
or
3 / 7
But 3 : 7 is most common :)