Answer:
a = 3 [m/s²]
Step-by-step explanation:
To solve this problem we must use the following equation of kinematics.

where:
Vf = final velocity = 60 [m/s]
Vo = initial velocity = 20 [m/s]
a = acceleration [m/s²]
t = time = 10 [s]
Now replacing these values we have:
60 = 20 + (a*10)
60 - 20 = 10*a
30 = 10*a
a = 3 [m/s²]