Help with linear equation code?

Question

The problem is described in more detail inside the code, but the basic idea is that when the button which toggles "+" to "-" is set to "-" the equations solving behaves very strangely. Can anyone help me with this

https://code.sololearn.com/W1V4vtl09Qgy/?ref=app

Example of bug (compare the two pictures):

https://uploadpie.com/sGYo8s

https://uploadpie.com/C88beB