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) Blank
(2) 3x - 4 = 2x + 16 Blank
(3) 3x = 2x + 20 Blank
(4) x = 20 ?

Question

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)               Blank
(2) 3x - 4 = 2x + 16               Blank
(3) 3x = 2x + 20                     Blank
(4) x = 20                                 ?

anonymous · Answer

the blank boxes are for you to plug in x=20 to prove its right.
so it would be 
3(20)-4=2(20+8)
60-4=2(28)
56=56
so its true!

anonymous · Answer

i need the reason =S

anonymous · Answer

you need the reason? i just proved that it was correct?

anonymous · Answer

http://prntscr.com/cojcl that is a similar image to what i need i neet the answer to the one under the question mark in the photo

anonymous · Answer

it has to be one of the propertyies: substitution adition multiplication etc... and i dont know what it is

anonymous · Answer

im pretty sure its the associative property of multiplication