import math

def quadratic_equation(a, b, c):

    if (b*b-4*a*c)>=0:

        x1=(-b+math.sqrt(b*b-4*a*c))/(2*a)

        x2=(-b-math.sqrt(b*b-4*a*c))/(2*a)

    return x1,x2

    else:

        return None

print quadratic_equation(2, 3, 0)

print quadratic_equation(1, -6, 5)