Answer:
x = 333.33 [m]
Step-by-step explanation:
To solve this problem we must use the following kinematics equation.

where:
Vf = final velocity = 0
Vi = initial velocity = 20 [m/s]
a = desacceleration = 0.6 [m/s^2]
x = distance [m]
Note: the final speed is zero as the body finishes its movement.
Now replacing:
0 = (20)^2 - (2*0.6*x)
1.2*x = 400
x = 333.33 [m]