Answer :
find the factors and get the ones common to both of them
45=3 times 3 times 5
81=3 times 3 times 3 times 3
so the factors common to both are 3 times 3 which is 9
45=3 times 3 times 5
81=3 times 3 times 3 times 3
so the factors common to both are 3 times 3 which is 9