Answer: If we have a set defined as the difference of two sets, this is
A = B - C, then A = B - B∩C
So this is defined as, if you subtract the set C to the set B, then you are subtracting the common elements between C and B, from the set B.
so if A = {a,b,c} - {a,b,d}, the common elements are a and b, so: A = {c}
