Can a Man Catch a Constantly Accelerating Train?

[Jess1986]

Please help with this problem. A train moves from rest with const accel. a in a straight line. A man at distance b behind the train chases it with const. speed V. Show he can catch it if V^2 >2ab, and find when he does so.
I have found formulas for distance traveled for each and equated these but cannot understand where the Vsquared comes from.
Thanks to anyone who can help. Jess x

 

[Doc Al]

Hint: When you equate those distance formulas you'll get a quadratic equation. What must be true for that quadratic to have a physically meaningful solution?