Speed of the slower truck is x and distance in 6 hours is 6x
Speed of the faster truck is 2x and distance in 6 hours is 12x
12x-6x=204
6x=204
x=34
The slower truck is traveling at 34 mph for 204 miles in 6 hours
The faster truck is traveling twice at 68 mph for 408 miles in 6 hours
The difference of this two miles is 204 miles