The third identity

If A = B, then B = A

A balances B. Swap them, and B balances A. It does not matter which side each one is on.

If one thing is equal to another, the other is equal to the first.

This identity is called the symmetric property of equality.

Symmetric means it works both ways: either side of the = can come first. We write it if a = b, then b = a.

OK