Lets first talk about a
since on prime factorisation it yeilds 3 as least prime no.,all other prime factors must be odd only since 2 is not there the resulting no. will be an odd no.( product of odds=odd)
Similarly in b 7 is the least prime no. So it must be an odd no. Sum of two odd no. is even i.e (a+b) is even .
So its prime factor (least) is 2