Using the highest common factor
The number of bunches must divide the number of white roses, the number of yellow roses and the number of red roses.
The highest common factor of $24$, $36$ and $42$ is $6$, so the most bunches Flora can make is $6$, each consisting of $4$ white, $6$ yellow and $7$ red roses.
Use all the roses to make identical bunches - so all bunches will have the same ratio of colours as the total
Cannot simplify further so each bunch has $4$ white roses
So Flora can make $6$ from $24$ white roses