She skied 2/3 of the distance of the trail which is the same as saying she skied 2/3 of the 3/4 mile.
To find this distance you multiply 2/3 and 3/4. Multiple across the top (numerators) and across the bottom (denominators) to get 6/12 or 1/2 mile.
You can also cross cancel the 2 and the 4 to make them 1 and 2 and then multiply straight across.