All you have to do to combine them is

c = a | b

Sets are unordered sequences of unique values. a | b or a.union(b) is the union of the two sets (a new set with all values found in either set). This is a class of operation called a "set operation", which Python sets provide convenient tools for.

You can use .update() to combine set b into set a. Try this:

a = {'a', 'b', 'c'}
b = {'d', 'e', 'f'}
a.update(b)
print(a)

To create a new set, c you first need to .copy() the first set:

c = a.copy()
c.update(b)
print(c)

You can use the result of union() of a and b in c. Note: sorted() is used to print sorted output

    a = {'a','b','c'} 
    b = {'d','e','f'}
    c=a.union(b)
    print(sorted(c)) #this will print a sorted list

Or simply print unsorted union of a and

    print(c)  #this will print set c

If you wanted to subtract two sets, I tested this:

A={'A1','A2','A3'}
B={'B1','B2'}
C={'C1','C2'}
D={'D1','D2','D3'}

All_Staff=A|B|C|D
All_Staff=sorted(All_Staff.difference(B)) 
print("All of the stuff are:",All_Staff)

Result:

All of the stuff are: ['A1', 'A2', 'A3', 'C1', 'C2', 'D1', 'D2', 'D3']