I really need help setting up the equation to solve the foll

I really need help setting up the equation to solve the following problem more than anything. Once I get there, hopefully I will be able to get it. Could you please help me set up the following word problem?
2 trains leave the depot at the same time, one traveling north and the other south. The speed of one train is 15 MPH slower than the other. If after 3 hours, the distance between trains is 315 miles, what is the speed of each train?

Solution

1st train DATA:
rate = x-15 mph ; time = 3 hrs. ; distance = rt = 3(x-15) miles
----------------------------------------
2nd train DATA:
rate = x mph ; time = 3 hrs. ; distance = rt = 3x
--------------------------------------
Equation:
distance + distance = 315 miles
3(x-15)+3x = 315
6x - 45 = 315
6x = 360
x = 60 mph (speed of the faster train)
x-15 = 45 mph (speed of the slower train)