If $\sin 2 \theta = \frac{21}{25}$ and $\cos \theta - \sin \theta > 0,$ then compute $\cos \theta - \sin \theta.$
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
If $\sin 2 \theta = \frac{21}{25}$ and $\cos \theta - \sin \theta > 0,$ then compute $\cos \theta - \sin \theta.$
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
\frac{2}{5}
If $\sin 2 \theta = \frac{21}{25}$ and $\cos \theta - \sin \theta > 0,$ then compute $\cos \theta - \sin \theta.$
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