There are several information's already given in the question. Based on those information's the answer can be easily deduced.
Distance covered by Terrance in 1/2 hour = 6 miles
So
Distance covered by Terrance in 1 hour = 6 * 2 miles
= 12 miles
So the unit rate of Terrance is 12 miles per hour.
Now
Distance covered by Jessie in 1/4 hour or 15 minutes = 2 miles
Then
Distance covered by Jessie in 1 hour = 2 * 4 miles
= 8 miles
Then the unit rate of Jessie is 8 miles per hour.