This is called the Pythagorean theorem : a ² + b ² = c ². You can substitute any of the variable with any of the known numbers and then you all you have to do is isolate the variable. I hope that helps!!
If one addend is negative, you basically subtract it from the other number
-13 + 13 basically equals 13 - 13 =0
I wanna say the 6th one down
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