Here!
[tex](x + 2 \sqrt{3} i)(x - 2 \sqrt{3}i )[/tex]
Let's see what complex numbers are!
[tex](x + 2 \sqrt{3 }i )(x - 2 \sqrt{3}i ) \\ = {x}^{2} - 2 \sqrt{3} x + 2 \sqrt{3} x - {(2 \sqrt{3} i)}^{2} \\ = {x}^{2} - {(2 \sqrt{3}i) }^{2} [/tex]
Now we know,
[tex] {i}^{2} = - 1[/tex]
So!
[tex] = {x}^{2} - 12 \times( - 1)[/tex]
[tex] = {x}^{2} + 12[/tex]
And thus we get the answer!