Answer:
30 N
Step-by-step explanation:
The buoyant force acting on an object is an upward force exerted by the fluid in which the object is immersed.
The magnitude of the buoyant force is equal to the weight of the fluid displaced by the object, mathematically:

where
is the density of the fluid
V is the volume of fluid displaced
g is the acceleration due to gravity
The formula can be written also as

where m is the mass of fluid displaced.
In this problem, the 3-Liter container filled with mercury is completely submerged in the water: this means that the volume of water displaced is
V = 3 L
We also know that 1 L of water weights about 10 N, so the weight of 3 L of water is

And therefore, this is the buoyant force acting on the container: 30 N.