Answer :

That is the lowest form of 3/7... you can not divide 3 by 7.. say the fraction was 2/4.. you would be able to take 2 divided by 4 and get 1/2. 
3/7 Is already in the simplest term because there is no common factor between 3 and 7 unless it's a decimal, but we don't use decimals in fractions. So 3/7 simplified is just 3/7!