Δs5=S5-S4=1/2×at5^2-1/2×at4^2=1/2×a(t5^2-t4^2)
2.1=1/2×a(25-16)
a=0.467m0.467m/s^2
S8=1/2×at8^2
=1/2×0.467×8^2
=14.944m