Adding similar elements basic misunderstanding $(10^n) + 9  (10^n)$

I don't understand the process behind converting this to 

$10 *(10^n)$

how does one logically get from the first step to the second.
Distribute:

$a + 9a = 1a + 9a = (1+9)a = 10a$.

So $10^n + 9*10^n = (1*10^n + 9*10^n) = 10^n(1 + 9) = 10^n*10 = 10^{n+1}$
$$(10^n) + 9  (10^n)=(10^n) + (10-1) (10^n)=???$$