Can someone explain me this property with an example? I've been trying to get it for 1/2 days

[tight quest]

I tried atributing values to the groups such as A = {1,2,3} B = {4,5,6} I just can't understand how C_a(A-B) = B

Was A-B supposed to be in the upper part of C_a? and if so wouldn't that make an empty group?