Cum se rezolva aceasta problema?

v=dx/dt=(-2t²+4t+4)'/t'=-4t+4
1 s≤t≤5 s=>t∈[1,5] s
v₁=-4+4=0 m/s
v₂=-4*5+4=-20+4=-16 m/s
vm=(v₁+v₂)/2=(0-16)/2=-16/2=-8 m/s
vm=-8 m/s