Given :
A={x:x is multiple of 3}
B={x:x is multiple of 5}
Therefore,
A={3,6,9,12,15,.....}
B={5,10,15,20,25....}
B' = {3,6,9,12,18, .......}
A∩B' = {3,6,9,12,18, ........}
Now A−B signifies all those elements of A that are not included in B.
A − B = {3,6,9,12,18, ........}
now,
A−B= A∩B'