they are
[tex] \frac{-b+ \sqrt{b^2-4ac} }{2a} [/tex] and [tex] \frac{-b- \sqrt{b^2-4ac} }{2a} [/tex]
we can see that we can add them because of the same denomenator and also we can see the square root bits cancel each other out
we are left with
[tex] \frac{-b-b}{2a}= \frac{-2b}{2a}= \frac{-b}{a} [/tex]
the sum is -b/a
