I took a slightly different approach and compared the distances and times of the green car in the two scenarios. It took the red car 5.33s in the first scenario to cross 43.2m from 0m, but it took the green car 5.33s to get to 43.2m from 213m: A difference of 169.8m. In the second scenario, it took the red car 4.77s to cross 76.8m from 0m, but it took the green car 4.77s to get to 76.8m from 213m, which took him 136.2m.
From this, we can find the velocities of the green car in each of these scenarios: 169.8m / 5.33s = 31.86 m/s, and 136.2m / 4.77s = 28.55 m/s. The difference in distances these scenarios are is 76.8m - 43.2m = 33.6m, and in 33.6m, his m/s changed 31.86 m/s - 28.55 m/s = A change of 3.31 m/s.
He changed 3.31 m/s in 33.6 m, so if we divide 33.6m by 3.31 m/s, we get that for every m/s he changed, he went 10.15 meters. But what we have here is m/s/m, not m/s/s, which is acceleration. So what I did (not sure if this is correct) to relate meters to seconds was to average the velocities in the two scenarios for the green car, which turned out to be 30.205 m/s. If there are 30.205 meters for every second, then for every 10.15 meters there are .336 seconds. Plugging this in, we get 1meter/second/.336seconds, which is the same thing as 2.976 m/s2, which is the constant acceleration.
If we want the initial velocity, we will want to work backwards from one of the scenarios. Since we had a 3.31 meters/sec change in 33.6 meters, then we can use this to find the initial velocity (at x = 213). For instance, at 76.8m, we know that the green car is moving at 28.55 m/s, and that we are 136.2m away from 213m. Since the change is 3.31 for each 33.6 meters, we can break up 136.2m into peices of 33.6m, which would be 136.2 / 33.6 = 4.054. But also, we are changing 3.31 m/s for each of these, and since we have 4.054 of them, we can just do 4.054 * 3.31 = 13.417 m/s difference as from x = 76.8. We know the difference is in the negative direction, so we can subtract 13.417 from 28.55 and get an initial velocity of 15.13 m/s.
Lemme know if I made any mistakes, i'm still trying to understand the basics.