Subtraction is finding the difference between two numbers whereas division is
putting numbers into equal pairs or undoing multiplication ( if that helps )
So...
If they say divide 6 by 2 they mean put six into equal pairs of 2 which would equal 3
E.g.:-
( there are 3 pairs of six cars in twos )
You can't use subtraction because that will just find the number difference of 6 and 2 which is 4
Hope that wasn't complicated