He did not solve for 5 variables, he solved 4 and the last one can be anything. It doesn't have to be in that relation. Example, 2 variable, 1 equation. x + y

He did not solve for 5 variables, he solved 4 and the last one can be anything. It doesn't have to be in that relation. Example, 2 variable, 1 equation. x + y - y = 0. Here x must be 0.