How can one come up with reasonable IP subnet allocations?
Hints:


Try working out the binary representation of 245.248.136.0 (it's 32 bits) and of 245.248.128.0.
Re-read what the /21 and /22 notation means and how CIDR works.
What is the range of IP addresses included in 245.248.128.0/22?  in 245.248.136.0/21?


I think once you understand the concepts you'll find that this question is straightforward.



Advice: Rather than asking a question about this specific exercise, you might do better to ask a question about some concept you don't understand: e.g., what /N notation means, how CIDR works, etc.  But make sure to do your research first.  There's lots written on CIDR, /N notation, etc.  You can probably clear up your confusion on your own by doing proper reading on those concepts.