To determine whether the expression \(6a - 14b + 3a - 6 + 21b - 18a\) is equivalent to \(-9a + 7b - 6\), the first property of algebra that should be applied is the Commutative Property.
The Commutative Property allows us to rearrange the terms in the expression, making it easier to combine like terms. After rearranging, we can combine the coefficients of like terms to simplify the expression.
So, the correct response is Commutative Property.