Question

can you tell me which of the following numbers is not a real number -/7 -7 /7 /-7

Answer 1

those slash marks mean square root?

if so, what is
\[ \sqrt{-7} \]

Answer 2

yeah

Answer 3

do you know what is
\[ \sqrt{9}= \]?
what about
\[ \sqrt{-9}= \]?