[tex]3 + \sqrt{ - 12} \\ = 3 + \sqrt{ 2 \times 2 \times - 3} \\ = 3 + 2 \sqrt{ - 3} [/tex]
Hope you could get an idea from here.
Doubt clarification - use comment section.
[tex]\\ \sf\longmapsto 3+√-12[/tex]
[tex]\\ \sf\longmapsto 3+√12i[/tex]
[tex]\\ \sf\longmapsto 3+√2(2)(3)i[/tex]
[tex]\\ \sf\longmapsto 3+2√3i[/tex]
