jmc

A terminating decimal can be written in the form $\frac{a}{10^b}$, where $a$ and $b$ are integers. So we try to get a denominator of the form $10^b$: $$\frac{21}{2^2\cdot 5^7}\cdot \frac{2^5}{2^5}=\frac{21\cdot 32}{10^7}=\frac{672}{10^7}=\boxed{.0000672}.$$