To divide fraction by number we can multiply fraction by inverse of number.
Lets find inverse
21=[tex] \frac{21}{1} [/tex] =====> [tex] \frac{1}{21} [/tex] - its inverse of 21
Now we can multiply it
[tex] \frac{9}{49}* \frac{1}{21}= \frac{9*1}{49*21} = \frac{9}{1029}= \frac{9:3}{1029}= \frac{3}{343} [/tex] - its our result