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.
This property allows you to rearrange the terms in the expression, which is helpful for grouping like terms together (like the \( a \) terms and \( b \) terms). Once the terms are rearranged, you can combine like terms to simplify the expression.
So, the correct response is:
Commutative Property