A quadratic solver

You will write the quadratic formula as a Python function and use it.

Your tasks.

1. In discriminant, return $b^2 - 4ac$.
2. In roots, return the list of real roots, the one with $+$ first. Return an empty list when the discriminant is negative.
3. Set answer to the roots of $2x^2 - 9x + 4 = 0$, using your function.