The electrons are not found in the nucleus. Electrons are in constant movement in the electron cloud. The only sub-atomic particles in the nucleus are the protons and neutrons.
Water displacement. You fill a graduated cylinder with an amount of water, place the object inside the graduated cylinder, and then measure the new water level. The change in volume of the water is the volume of the object, assuming the object was completely submerged.
Hey!
First, let's write the problem.

Subtract the numbers, we would do the following operation,


Add 2 to both sides.

This tells us that our final answer would be,

Thanks!
-TetraFish
Answer: Some challenges that I could see would be the walls, possibly a moat, tar, and the towers.
Explanation: The wall is obliviously a main problem, trying to get over it or through it is a difficult challenge. The moat (if it has one) means that there is more than likely only one way to get in or out. If it does have tar it means that the attackers are going to be put in a "sticky situation" And finally the towers, they have people at the top shooting arrows down at you, or throwing things at you.
The answer is electric current