Hi there! The answer is 6x
[tex]4 x^{2} - 8x +8 - x^{2} +2x - 4 - 3 x^{2} + ? = 4[/tex]
[tex]0 x^{2} - 8x +8 +2x - 4 + ? = 4[/tex]
[tex]- 8x + 4 + 2x + ? = 4[/tex]
[tex]- 6x + 4 + ? = 4[/tex]
Now we can see we have to add 6x to make the equation correct.
