First day trip was 70 miles longer than second day trip.
Explanation:
Total trip length = 350 miles
Jack completed 3/5 of his two day trip in the first day.

Distance covered in second day = 350 - 210 = 140 miles
Difference between distance covered in first day and second day = 210 - 140 = 70 miles
First day trip was 70 miles longer than second day trip.