Answer:
a = -1,33 m / s²
Explanation:
This problem is kinematic in one dimension, let's use the equations
v = v₀ + a t
v₀ = v - at
x = v₀ t + ½ a t²
we substitute the first in the second equation
x = (v-at) t + ½ a t²
x -v t = a t² (1 + ½)
a = (x -vt) / t² (1.5)
we calculate
a = (100 - 30 10) / 10² (1.5) =-200 / 150
a = -1,33 m / s²