Maths problem: how to do, thanks?

v1 = 72 km/h = 20 m/s
v2 = 36 km/h = 10 m/s
動能減少
     (1/2)×1500kg×(20^2-10^2)(m/s)^2
        = 225KJ
假設動能減少全是由於減速所作負功,
    225KJ = 9000N．s
∴ s = 225kJ/9000N = 25m

另法, 由 F = ma,
∴ a = 9000N/1500kg = 6m/s^2
以等加速度計算, 由 v1 減速至 v2, 花費時間
    t = (20m/s-10m/s) ÷ 6m/s^2 = 5/3 s
這段時間位移是
    20t - (6/2)t^2 = 20(5/3) - 3(5/3)^2 = 25 (m)