Just think of it that it is like this:
13-squared x r-squared -a-squared
169 x r-squared a-squared
Now, WHY DID -a-squared become positive a-squared?
Answer: Given -3-squared:
We all know that -3-squared is 9.
But, it is just the same with 3-squared and both of them yields 9.
So, simple, we just simplified it, done!