v1=s/t v2=s/(t+4) s=210km
V=v1-3=v2=3
s/t - 3 =s/(t=4) +3
s/t -6=s/(t+4)
(s-6t)/t=s/(t+4)
(210-6t)/t=210/(t+4)
(210-6t)(t+4)=210t
210t+840-6t²-24t=210t
6t²+24t-840=0
6(t²+4t-140)=0
t²+4t-140
delta=4²-4×1×(-140)=16+560=576; √576=24
t1=(-4-24)/2=-14-nie należy do rozw bo -
t2=(-4+24)/2=10
zatem v=v1-3=210/10 -3=21-3=18
odp v=18 km/h.
