The answer is not 2 hours.
The trains will meet up when the first train has been going for 3 hours and the second train has been going for 2.5 hours. Each will have travelled 180 miles at that point in time.
t is the time that the first train has been traveling in hours.
t-0.5 is the time that the second train has been traveling in hours.
Distance that the first train travels during that time is
Distance that the second train travels is
The two trains are at the same distance when those two expressions are equal
t*60 = (t-0.5)*72
60t = 72t-36
36 = 12t
3 = t
I hope that helps.