first
second
and fifth are true
Basically the square root of -1 is i. From that you should be able to deduce the answers.
3 x 7 because three, seven times is 21
81 is a counter example of the conditional statement given.
81 has factors other than itself, 1 and the square root of 81 which is 9.
81 has factors of 3 and 27 which are not listed.
