Very basic stuff from school we know that we can calculate zero places of quadratic function which has form $ax^2 + bx + c$ and we assume that $a \neq 0$, now what if $a=0$? Why can't we use delta to calculate zero places of linear function? I know it's very easy to calculate zero place of linear function, but i'm quite interested in such stuff. Thanks for any response.
EDIT : Right, to make my question more understandable: From where such formula of roots of quadratic function come from? And as interesting as possible, elaborate this topic! i'll be greatly grateful.