The greatest common factor of both x^4 and x^3 would be x^3, since the largest number of X variables that you can evenly take out between both terms is x^3 or x cubed.
So the union of sets A and B is the set of elements in A, or B, or both. The symbol is a special "U" like this: ∪ Example: Soccer = {alex, hunter, casey, drew}