Answer 1

1. explaining null

you tried to access DOM before it exists.

https://www.sololearn.com/post/90825/?ref=app

https://www.sololearn.com/post/233784/?ref=app

https://code.sololearn.com/WKKkpq0efxai/?ref=app

2. fix:

https://code.sololearn.com/Wyit05d0LNvi/?ref=app

3. your other mistake:

use value to get value of input element

typo txt2 not txt1 for second input