Solving for i is outside the radical .
[tex]\\ \sf\longmapsto ab[/tex]
[tex]\\ \sf\longmapsto (5+\sqrt{3}i)(5-\sqrt{3}i)[/tex]
[tex]\\ \sf\longmapsto 5^2-(\sqrt{3}i)^2[/tex]
[tex]\\ \sf\longmapsto 25-3i^2[/tex]
[tex]\\ \sf\longmapsto 25-3(-1)[/tex]
[tex]\\ \sf\longmapsto 25+3[/tex]
[tex]\\ \sf\longmapsto 28[/tex]