To determine if 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.
This property allows you to rearrange the terms in the expression to group like terms together, which is essential for combining terms to simplify the expression.
Therefore, the correct response is:
Commutative Property