The 'i' should go in front of the square root, not inside it.
So the answer should be [tex]i\sqrt{15}[/tex]
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The way they wrote the expression [tex]\sqrt{15}i[/tex] is bit misleading as it might imply that the 'i' is under the square root. However the square root ends after the "5". It might be better to say [tex](\sqrt{15})i[/tex] so there is clearer separation. But sticking the "i" in front is also a valid option.
side note: the plus minus part is already handled so there's no need to worry about that.
