Answer:
-a/(10-a)
Explanation:
To get from a - 10 to 10 - a, you just have to multiply by -1.
So fi you multiply both the top and bottom by -1, you'd get -a/(10-a).
-a
-------
10-a
a
----
a-10
Multiply the top and bottom by -1
-1*a
-1(a-10)
------
-a+10
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