Why do u think rates are usually written as unit rates

Simonne used the following steps to simplify the given expression. 12-3(-2x+4) step 1: 12+(-)+(-3)(4) step 2: 12+6x+(-12) step 3: 12+(-12)+6x step 4: 0+6x step 5: 6x what property of real numbers was used to transition from step 3 to step 4? •identity property of addition •inverse property of addition •associative property of addition •commutative property of addition