This question belongs to greatest common factor.
49 = 7*7
56 = 7*8
So Greatest common factor = 7
Hence Susan can arrange 7 mugs on one shelf.
For 49 blue mugs, Susan can arrange 7 mugs in 7 shelves.
For 56 red mugs, Susan can arrange 7 mugs on 8 shelves.
Hence, 7 shelves are needed for BLUE mugs and 8 shelves are needed for RED mugs.