Answer:
The yellow flower is dominant.
Step-by-step explanation:
If it wasn't, the offsprings would be all white flowers. Now let's say "what if both purebreds were both DOMINANT?" well my friend, then it's most likely that all the flowers wouldn't be yellow. Remember, the dominant trait takes over. If i remember correctly, purebred is like... pfft- for example, since i'm not good at explaining things properly, purebred is like BB or bb. I don't think it's Bb. If you know what i mean, good for you. :) I hope this helps! \(^-^)/