Answer:
The answer is 0.39 m/s ²
Step-by-step explanation:
Since the box moves with steady speed, 
 Friction=force in even direction=260*Cos20=244.3N 
 Ordinary force=mg+FSinθ=1089N 
 μN=f 
 μ=244.3/1089=0.224 
 N=mg-FSinθ=911.1N 
 a=(260 Cos20-0.224*911.1)/(1000/9.8)=0.39m/s²