Answer :
I think C. (-x-9i)(x-9i)
You can multiply them to get:
(-x-9i)(x-9i)=[tex]- x^{2} +9xi-9xi+81 i^{2} [/tex]=
remembering that: [tex]i= \sqrt{-1} [/tex] so that:
[tex] i^{2} =-1[/tex]
Finally:
[tex]- x^{2}+81 i^{2} [/tex]=[tex]= -x^{2} -81[/tex]
You can multiply them to get:
(-x-9i)(x-9i)=[tex]- x^{2} +9xi-9xi+81 i^{2} [/tex]=
remembering that: [tex]i= \sqrt{-1} [/tex] so that:
[tex] i^{2} =-1[/tex]
Finally:
[tex]- x^{2}+81 i^{2} [/tex]=[tex]= -x^{2} -81[/tex]