Answer:
A. X
Explanation:
You want to know which graph is decreasing for x > 3.
Decreasing
A graph is decreasing when it slopes down to the right. The only graph in the group that has any decreasing portion is graph X. The decreasing part of that graph is to the right of x = 3, where x > 3.
Graph X is the one you want.
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Additional comment
All the other graphs are increasing everywhere, so aren't even worth any consideration.