Alright, you got it! Here's my reasoning: subscript 1 denotes quantities related to the acceleration part of the trip down the runway. Subscript 2 denotes quantities related to the deceleration part of the trip. Let d be the total length of the runway, then 
            Acceleration                   Deceleration
      v_1² = 0 + 2a_1(3/4 d)         v_2² = v_1² - 2a_2(1/4 d)
           = 3/2 a_1d    ==>          0   = 3/2 a_1d - 1/2 a_2d  ===>
                                      a_2 = 3a_1 = 3(2.4 m/s² ) = 7.2 m/s²

larryg@upenn5.hep.upenn.edu
Sat Jul 29 18:57:40 EST 1995
ans3_subsection1_1_2.html