There are 7 girls on a bus 7*2=14
Each girl has 7 backpacks
In each backpack, there are 7 big cats 7* 7*7=343*4=1372
For every big cat there are 7 little cats 7*7*7*7=2401*4=9604
Total number of legs=14+1372+9604=10990
See if this works and let me know?