The Starship Enterprise returns from warp drive to ordinary

Enterprise's speed is V1=53km/s
Klingon's speed is V2=20km/s
let's suppose Enterprise's acceleration ：a=?km/s^2
suppose accelere time：t=?s
an Uniformly retarded rectilinear motion
→with Enterprise S1=V0t+1/2at^2=53*t+1/2at^2 ①
→with Klingon S2=Vt=20*t②
to barely avoid a collision
→S1=S2+140KM③
solve①,②and③
a≈-3.8893km/s^2
Anather way
if Enterprise can reduce the speed to 20km/s in the 140km,it can barely avoid a collision!
so this question became → S=1/2at^2
use average speed → V=[0+（53-20）]/2=16.5km/s
so S=V(average speed)*t → t=140/16.5
so acceleration a=△V/t=(-33)/(140/16.5)=-3.8893