What belongs in the box marked with the question mark in the proof? given: 3x - 4 = 2(x + 8), prove: x = 20 1. 3x - 4 = 2(x + 8) | 1. 2. 3x - 4 = 2x + 16 | 3. 3x = 2x + 20 | 3. 4. x = 20 | 4. ( what is the process called like what did they do to 3 to get 4 )