solution: Since $10^x - 10 = 9990,$ we have $$10^x = 9990+10=10000.$$If $10^x = 10000,$ then $x=\boxed{4},$ since $10000$ ends in four zeroes.
prompt.content: <|im_start|>system
  You are a helpful assistant.<|im_end|>
  <|im_start|>user
  If $10^x - 10 = 9990,$ what is $x$ equal to?
  Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
  <|im_start|>assistant
  
prompt.role: user
extra_info.answer: 4
extra_info.question: If $10^x - 10 = 9990,$ what is $x$ equal to?
extra_info.split: test