**AP Computer Science Quiz**

In algebra, for polynomials of the form *ax ^{2} + bx + c = 0, *the
solutions for

In Scheme, we can take two numbers and put them into a list by using the `list`
function: (list num1 num2).

Write a function called `quadratic` which has three parameters, `a`,
`b`, and c. It returns a list of all possible solutions to *ax ^{2}
+ bx + c = 0*.

For this problem, assume that a solution means a real number, so if the discriminant
(*b ^{2} - 4ac*) is less than zero, your program should return
an empty list. If the two solutions are the same, the list should contain the
one solution, not two instances of the same solution.

Examples:

> (quadratic 1 0 -1) ; x^{2}- 1 = 0 (1 -1) > (quadratic 1 5 6) ; x^{2}+ 5x + 6 = 0 (-2 -3) > (quadratic 1 -2 1) ; x^{2}- 2x + 1 = 0 (1) > (quadratic 1 2 2) ; x^{2}+ 2x + 2 = 0 ()

Note that the order of results does not matter. `(quadratic 1 5 6)`
could just as easily have returned `(-3 -2)`.

Write `quadratic`.

Hint: You might first wish to write a function called `discriminant`
which returns *b ^{2} - 4ac*.