I'm going to answer this by using rounded numbers for the atomic masses. You need only go back and put the numbers in from your periodic table. My answers will be close, but not what you should get.
Find the Molar Mass of MgCl2
Mg = 24 grams
2Cl = 2 * 35.5 = 71 grams
Total = 95 grams
Find the mols in 339 grams
1 mole = 95 grams
x mol = 339
Solve
339 = 95x Divide by 95
339/95 = x
x = 3.67 mols Answer
W=F*s
W=9000*0
W=0
W:Work
F:Force
s:Distance
The force exerted on the box is 56 N
Explanation:
The work done by a force on an object is given by

where
F is the magnitude of the force
d is the displacement of the object
is the angle between the direction of the force and of the displacement
For the box in this problem, we have:
W = 2240 J is the work done
d = 40 m is the displacement of the box
Assuming that the force is parallel to the displacement, 
Solving the equation for F, we find the force exerted on the box:

Learn more about work:
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Answer:
It makes sense because on that the day the sun stops moving northward and starts moving southward
Explanation:
It lets the viewer know it's something to do with underwater.