Question 319172
You know that at the moment they meet, they've traveled the same distance, so you'll set the distances equal to each other for each train. Remember that rate(time) = distance. 

Let x = rate of passenger train

Passenger train: x(12.75)
Freight train: (x-6)(15.75)

12.75x = 15.75x - 94.5; x = 31.5 mph