If those 4 numbers average to 9, they must sum to $4\times 9 = 36$.  Then, we simply subtract the other three numbers from 36: $36 - 7 - 2 - 10 = \boxed{17} = x$.
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
The arithmetic mean of 7, 2, $x$ and 10 is 9. What is the value of $x$?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
17
The arithmetic mean of 7, 2, $x$ and 10 is 9. What is the value of $x$?
test