What's the answer for this?
explain why it's right

 
You can start by replacing the box with a Y just as a placeholder. Then try to simplify as much as possible

4(yx + 7) = 2(x+20) - 5(3-2x) +3
4*y*x + 28 = 2x+40 - 15+10x+ 3
4*y*x +28 = 12x+28
if we look closely now, we can see that if we set y as 3, we will have 
12x + 28 = 12x + 28

This works because, If you make both sites equal, it shouldnt matter what x you put in, as both 12's would be multiplied by the same value.

Hence the answer is 3 (or in this case, D).