They travelled 3 and 5/6 miles, then drove 2 and 2/6 more the next day. To find the total, you must add them up. So...
[tex]3 \frac{5}{6} + 2 \frac{2}{6} = 5 + \frac{7}{6} = 6 \frac{1}{6} [/tex]
They travelled for 6 and 1/6 hours, or 6 hours and 10 minutes.