Assuming the yellow, true-breeding plant has genes YY, and the green, true-breeding plant has genes gg, the F1 offspring would be Yg. I have made the Y's capitals because we are told the F1 offspring is yellow, suggesting the yellow gene is dominant to the green. Possible offspring for an F1xF1 cross would be:
Y| YY Yg
g| Yg gg
On average, out of four offspring, you would expect 1 YY, 2Yg, and 1gg.
The phenotypes given for these genotypes are YY-yellow
So out of 4 offspring, on average, you would expect 3 to be yellow, and 1 green.
I hope this answer helped you!