one of the steps dave used to solve an equation is shown below.
3x+9=21
3x+9-9=21-9

which statement describes the procedure dave used in this step and identities the property that justifies the procedure?

a) dave rearranged the terms. this procedure is justified by the commutative property.

b) dave subtracted 9 from both sides. this procedure is justified by the additive property of equality.

c) dave rearranged the terms. this procedure is justified by the associative property.

d) dave subtracted 9 from both sides. this procedure is justified by the additive inverse.