Решение квадратного уравнения и вывод его корней (ошибка Runtime Error)

import math
a = float(input())
b = float(input())
c = float(input())
x1 = None
x2 = None
x3 = None
x4 = None
x5 = None
x6 = None
x7 = None
x8 = None
x9 = None
if a != 0 and b != 0 and c == 0:  # свободный члне уравнения равен 0
    x1 = (-b)/a
    x2 = 0
    print(x1, x2)
elif a != 0 and b == 0 and c != 0:  # коэффициент b = 0
    if (a > 0 and c > 0) or (a < 0 and c < 0):
        print('')
    else:
        x3 = math.sqrt(abs(c / a))
        x4 = x3 * -1
        print(x3, x4)
elif a != 0 and b == 0 and c == 0:
    x5 = 0
    print(x5)
elif a != 0 and b != 0 and c != 0:
    d = math.sqrt((b ** 2) - (4 * a * c))  # Расчет дискриминанта
    if d > 0:
        x6 = (-b + d)/(2 * a)
        x7 = (-b - d)/(2 * a)
        if x7 > x6:
            print(x6, x7)
        else:
            print(x7, x6)
    elif d == 0:
        x8 = (-b)/(2*a)
        print(x8)
    elif d < 0:
        x9 = None
        print(x9)

import math
 
a = float(input("a:"))
if a == 0:
    print("a shouldn't be 0 according to task conditions")
else:
    b = float(input("b:"))
    c = float(input("c:"))
    D = b ** 2 - 4 * a * c
    x1 = (-b + math.sqrt(D)) / (2 * a)
    x2 = (-b - math.sqrt(D)) / (2 * a)
    print("X1:", x1, "X2:", x2)

import math
 
a = float(input("a:"))
if a == 0:
    print("a shouldn't be 0 according to task conditions")
else:
    b = float(input("b:"))
    c = float(input("c:"))
    D = b ** 2 - 4 * a * c
    if D > 0:
        x1 = (-b + math.sqrt(D)) / (2 * a)
        x2 = (-b - math.sqrt(D)) / (2 * a)
        print("X1:", x1, "X2:", x2)
    elif D == 0:
        x1 = x2 = (-b ) / (2 * a)
        print("X1:", x1, "X2:", x2)
    else:
        print("there is no solution")