simplify 4a/7ab?

Question is wide open to doubt due to lack of brackets.
Could it be that you mean :-

[ 4a ] / [ 7ab ] = 4 / [ 7 b ]

4/7b

4a/7ab = 4/7b
a cancels out

4/7b.

4a/7ab

a/a(4/7b)
1(4/7b)=4/7b

the a on top and bottom cancel out leaving 4/7b