import math


def find_roots_quadratic(a: float, b: float, c: float) -> set:
    """Return a set containing the solutions to the equation ax^2 + bx + c = 0.

    Each solution is a float.

    You may ASSUME that:
      - a != 0
      - (b * b) - (4 * a * c) >= 0

    >>> find_roots_quadratic(1, -15, 56) == {8.0, 7.0}
    True
    >>> find_roots_quadratic(1, -10, 21) == {3.0, 7.0}
    True
    >>> find_roots_quadratic(1, 8, 15) == {-3.0, -5.0}
    True
    >>> # Have to use isclose to compare floats
    >>> all([math.isclose(sol, -0.739, abs_tol=0.001) or math.isclose(sol, 1.739, abs_tol=0.001) for sol in find_roots_quadratic(7, -7, -9)])
    True

    Hint: use the quadratic formula.
    """
    assert a != 0
    assert (b * b) - (4 * a * c) >= 0
    part = math.sqrt(b * b - 4 * a * c)
    return {(-b - part) / (2 * a), (-b + part) / (2 * a)}