If the solution of two equations is like 0=0, then the equations are equilateral with infinite common solution.
Why is this case different from 0=0? I assume you mean x = 2 or y = 7.Rule 3:
If the solution of two equations is like 2=2 or 7=7 or 9=9, then the equations have only one solution.
Don't memorize so many rules that you forget to think about what you are doing?