That is complicated. A easier way to do......
No matter how you select the flowers, you must have selected a red flower or a yellow flower, so the probability of selecting a flower(red or yellow) is 1.
P(Red) + P(Yellow) = 1
I guess you wouldn't have found a turqoise coloured flower out from nowhere? <- That's not funny. You laugh because you sucks.
7/15 + P(Yellow) = 1
P(Yellow) = 1 - 7/15 = 8/15
Sorry I am a bit moody today so that I am not very nice. Even to my father and mother.