how can i use complex number in this online scientific calculator

Dear Will85237,

If you study mathematics long enough you will find out about imaginary numbers.

Imaginary numbers as based on i where     $$i= \sqrt{-1}$$

Complex numbers have a real number part and  an imaginary number part  like  6+5i

There you go, you have learned something new   

Either this is some branch of mathematics that is above me, or this post isn't detailed enough.

Can you give an example of a complex number?

example:

$${\mathtt{2}}{i}{\mathtt{\,\times\,}}{\mathtt{2}}{i} = -{\mathtt{4}}$$