The Book Club Is Here! !

Created a book friend group, the group number is: 7-0-6-8-0-0-5-8-9

There will be a problem when joining a group, and the problem is below.

Well... everyone, please listen to the question! It's time to test whether you are a top student!

………………

Here is a system of homogeneous linear equations

X1+X2+2X3-X4=0

2X1+X2+X3-X4=0

2X1+2X2+X3+2X4=0

The rank of the coefficient matrix of this equation system is the answer to the group! !

Welcome everyone to join the top academic exchange group! ! Warm welcome! !