Too many tutors blurt out solutions without explaining why.
You asked this:
7/12 are red roses?
No. 7/12 would have been the answer to this question:
What fraction of the roses are red roses?
But that was not the question. The question was:
What fraction of the flowers are red roses?
Since the roses are only 2/3rds of the flowers,
the red roses are 7/12ths of 2/3rds of the flowers.
So we take 7/12th of 2/3rds, which means that we
multiply the two fractions:
Edwin