So here you want to convert both of these numbers until they have the same denominator.
The LCM of the two is 10, so the closest same denominator of the 3 4/5 and 3 1/2 is 10.
3 4/5 -> 3 8/10
3 1/2 -> 3 5/10
The difference between the two is 3/10 feet.
She will have to cut 3/10 feet of the 3 4/5 length rope for the two to be the same length.