Hello there!
As we have 5 green picks for every 2 orange picks, we can use this as a function.
2x + 5y = 21
First, let's just count up by our oranges to make things easier.
2 oranges, 5 greens = 7 total
4 oranges, 10 greens = 14 total
6 oranges, 15 greens = 21 total.
Since we needed 21 in total, let's review what we came to with our answer.
With every 2 oranges, 5 greens are picked. This means every 4 oranges, there is 10 green, and for every 6 oranges, there is 15 green.
Your answer is 6 oranges.
I hope this helps!