The property used in line A is the commutative property
This is the idea of adding can be done in any order, eg: 2+3 = 3+2 = 5
For this problem, 2x-20+x turns into 2x+x-20. Think of 2x-20 as 2x+(-20).
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The property you would use after line B is the addition property of equality
This rule says that if a = b, then a+c = b+c. In words: we can add any number to both sides of any equation. In this case, we would add 20 to both sides to undo the "-20" on the left side.
