If anyone wants a TLDR, here’s my try.
We use the kinematics equation VF2 = 2AS + V02
where VF is final velocity, V0 is initial velocity, A is acceleration, S is distance.
The first car decelerates (A) from 70 (V0) to 0 (VF) in distance S, so 02 = 2AS + 702
We dont know what the distance (S) is, nor do we know what the rate of deceleration (A) is. We dont need to know. All we need to know is 2AS = -702
The second car decelerates (A) from 100 (V0) to VF in same distance S, so VF2 = 2AS + 1002 = -702 + 1002
VF = √(-702 + 1002) = √5100 = 71.4
