How do you solve the system y = 2/3x - 7 and 2x - 3y = 21?
There is no single solution to the system of equations.
Suppose we attempt this one "by substitution"
Whoa! we ended up with a true equation but no variable solution!
What does this mean?
It means that the original equations were not independent; they are just rearranged versions of each other.
In terms of a graph the two equations represent the same line and therefore do not intersect to give a single solution.