Total number of subsets of the first is 2m while total number of subsets of the second is 2n.
Now it is given that 2m=2n+56 ⇒2m−2n=56 ⇒23(2m−3−2n−3)=56 ⇒(2m−3−2n−3)=7 ⇒(2m−3−2n−3)=8−1 ⇒(2m−3−2n−3)=23−20 Hence compare the terms, m−3=3 thus m=6 and n−3=0 giving n=3 Hence, (m,n)=(6,3)