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exponential simplification

Why does

10^x - 10^(x - 1)

simplify to

9 * 10^(x - 1)

?

It's not homework, it's from a book of combinatorics, explaining why
the number of k-digit positive integers is

9 * 10^(k - 1)

Can someone walk me through this?  I'm just not following.

Thanks for any assistance.

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