I'm working with palindromes from $100-999$. I'm having trouble with the step highlighted in red. Can someone explain the algebra to me?

Taken from: Discrete and Combinatorial Mathematics: An Applied Introduction, by Ralph P. Grimaldi, 5th edition.
I'm working with palindromes from $100-999$. I'm having trouble with the step highlighted in red. Can someone explain the algebra to me?

Taken from: Discrete and Combinatorial Mathematics: An Applied Introduction, by Ralph P. Grimaldi, 5th edition.
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