I think that I have misunderstood the first reply of Teacher Tdol, which may probably be caused by our culture difference.
Though I almost forgot all the algebra formulas, as a rule, I still insist that:
One unknown in one equation
Two unknowns in two equations
Three unknowns in three equations
Those unknowns can be solved!
I normally don’t call myself a liar, but sometimes an imbecile.