To determine whether \( 6a - 14b + 3a - 6 + 21b - 18a \) is equivalent to \( -9a + 7b - 6 \), the first property of algebra that should be applied is the Commutative Property.
The Commutative Property allows you to rearrange the terms in an expression. For example, you can change the order of addition to combine like terms more easily. Once the terms are rearranged, you can then combine like terms to simplify the expression.
So, the correct response is:
Commutative Property