In 40 seconds, the train reduced its speed from 30 m / s to 10 m / s. what path did he forgive during this time

In 40 seconds, the train reduced its speed from 30 m / s to 10 m / s. what path did he forgive during this time and with what acceleration did he move?

Given:
t = 40c
Vo = 30
V = 10

To find:
t-?
a-?

Decision:
a = (V-Vo): t
a = (10-30): 40
a = (- 20): 40
a = -0.5 (m / s ^ 2)

S = (V ^ 2 – Vo ^ 2): 2a
S = (100-900): (- 1)
S = (- 800): (- 1)
S = 800 (m)

Answer: a = -0.5 (m / s ^ 2), S = 800 (m)