Answer:
m = 0.305 [kg]
Step-by-step explanation:
When a body rests on a horizontal surface and is in balance we can say that normal force is the reaction of the weight of a body.
Weight is defined as the product of mass by gravitational acceleration.

where:
N = normal force = 3 [N]
g = gravity acceleration = 9.81 [m/s²]
m = mass [kg]
Now replacing:
![3=m*9.81\\m=0.305[kg]](https://img.qammunity.org/2022/formulas/physics/college/56y6vdon37yb4zsnaeiwdezmi70whr0k7f.png)