Which of the following numbers belong to the set of complex numbers and the set of imaginary numbers? select all that apply.

3i
-5i
6-i
square root of 21
negative square root of 16
square root of -14

A complex number is a number of the form a+bi, 

if b=0, that is if the number is a, then it is a Real number

if a=0, that is if the number is bi, then it is called an imaginary number.

All imaginary numbers are complex numbers.

Also, all real numbers (non-imaginary) are complex numbers.

Thus, all the numbers given are complex.

3i   :   imaginary

-5i :    imaginary

6-i:    complex

square root of 21: real, non-imaginary, complex

Negative square root of 16=-4:  real, non-imaginary, complex

Square root of -14 = (square root of 14)i:   imaginary