Answer:
 500kg. 
Step-by-step explanation:
Newton's Second Law relates force, mass and acceleration to one another through the equation: 
F = ma 
We can rearrange this equation algebraically to solve for mass:
m = F/a
Substituting our known values for the variables, we get: 
m = 1500N/3m/s2
So mass of the object is 500kg. 
We know it's kg because Newtons (N) are a unit defined as 1 kg/m/s2, so m/s2 cancels out when we divide :)