During a race, one runner runs at a 6 min/mi pace and another runner runs at a 7 min/mi pace. The fast runner passes the slow runner. After a 2 second delay, the slow runner initiates an acceleration, catching up with the fast runner in 968 feet. Give this acceleration in ft/sec^2.

Question

During a race, one runner runs at a 6 min/mi pace and another runner runs at a 7 min/mi pace.  The fast runner passes the slow runner.  After a 2 second delay, the slow runner initiates an acceleration, catching up with the fast runner in 968 feet.  Give this acceleration in ft/sec^2.

I have figured out that after the 2 seconds when the fast runner passes the slow runner, there are 4.19047 feet between the runners.  I got this by:
5280 ft/360 sec = x ft/2 sec   and
5280 ft/420 sec = x ft/2 sec  then finding the difference in the distance the runners had travelled over those 2 seconds.  However, I can't figure out what to do now.