Find the units digit of $n$ given that $mn = 21^6$ and $m$ has a units digit of 7.
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Find the units digit of $n$ given that $mn = 21^6$ and $m$ has a units digit of 7.
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
3
Find the units digit of $n$ given that $mn = 21^6$ and $m$ has a units digit of 7.
train