\magnification=\magstep1
$$ax^2 + bx + c = 0$$

$$a\left(x^2 + {b\over a} x + {c \over a}\right) = 0$$

$$\left(x + {b\over 2a} \right)^2 - {b^2\over 4a^2} + {c\over a} = 0$$

$$\left(x + {b\over 2a} \right)^2 - {b^2 - 4ac\over 4a^2} = 0$$

$$x + {b\over 2a}  = {\pm \sqrt{b^2 - 4ac}\over 2a}$$

$$x = {-b \pm \sqrt{b^2 - 4ac}\over 2a}$$

\bye
