Robin would need 2/3 cup for each of the flowers, so you can do this:
2 + 2
3 3
Because you have two flowers, you need two 2/3 cups of water. This is all about adding fractions together.
If the bottom numbers are equal (in this problem, the bottom numbers are the 3s), then you keep the bottom number the same, and add the top two numbers. This is what it looks like:
2 + 2 = 4
3 3 3
The bottom number always stays the same when you add, and the top numbers are what change.