I think you might have to factor in limits in this equation. I think it's not so much
undefined, but the proper term is indeterminate, I believe. If you factor in limits in the above equation and let the limits of
x approach zero, then you effectively illustrate that the equation is unsolvable algebraically, thereby removing the conundrum that has been put in place by attempting to solve the equation further.
"...To avoid this little inconsistency the epsilon-delta limit device is used to establish that the LIMIT of the ratio dy/dx is 2ax + b as dx APPROACHES zero."