Simonne used the following steps to simplify the given expression.

12 minus 3 (negative 2 x + 4)

Step 1: 12 + (negative 3) (negative 2 x) + (negative 3) (4)
Step 2: 12 + 6 x + (negative 12)
Step 3: 12 + (negative 12) + 6 x
Step 4: 0 + 6 x
Step 5: 6 x

What property of real numbers was used to transition from step 3 to step 4?