Given $ab = 21-20i$ and $|b| = 29$,  find $|a|.$
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Given $ab = 21-20i$ and $|b| = 29$,  find $|a|.$
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
1
Given $ab = 21-20i$ and $|b| = 29$,  find $|a|.$
train