Easy, find a common denominator, for
3/8
and
3/6
8: 8,16,24,
6:6,12,18,24
then take,
3/8*3 3*3=9 8*3=24
=9/24
then,
3/6*4 3*4=12 6*4=24
=12/24
Set them equal to each other, 3/24=12/24, which one is larger?
12/24, which is 3/6.
Thus,your answer, is 3/6 is longer.