初学者，编写了一段代码，可以跑，但有错误

我是一个初学者，下图是本人编写的一个求一元二次方程根的代码，为什么只有一个该方程无解可以显示？求求各位大神了
int a, b, c;
cout << "请依次输入该一元二次方程的二次项系数、一次项系数、常数项。" << endl;
cin >> a >> b >> c;
cout << a << "x*x+" << b << "x+" << c << "=0" << endl;
int d = b * b - 4 * a*c;
if (a = 0) {
double p = -c / b;
cout << "x=" << p << endl; }
else if (a = !0) {
if (d < 0) {
cout << "该方程无实数根" << endl; }
else if (d = 0) {
double x = -b / 2 * a;
cout << "该方程有两个相同的实数根x1=x2=" << x << endl; }
else if(d > 0) {
double x1 = (-b - sqrt(d)) / 2 * a;
double x2 = (-b + sqrt(d)) / 2 * a; cout << "x1=" << x1 << ",x2=" << x2 << endl; }
}
system("pause");
return 0;

#include<stdio.h>
#include<string.h>
#include<iostream>
using namespace std;


int main()
{
    double a, b, c;
    cout << "请依次输入该一元二次方程的二次项系数、一次项系数、常数项。" << endl;
    cin >> a >> b >> c;
    cout << a << "x*x+" << b << "x+" << c << "=0" << endl;
    double d = b * b - 4 * a * c;
    if (a == 0) {
        double p = -c / b;
        cout << "x=" << p << endl;
    }
    else if (a != 0) {
        if (d < 0) {
            cout << "该方程无实数根" << endl;
        }
        else if (d == 0) {
            double x = -b / 2 * a;
            cout << "该方程有两个相同的实数根x1=x2=" << x << endl;
        }
        else if (d > 0) {
            double x1 = (-b - sqrt(d)) / 2 * a;
            double x2 = (-b + sqrt(d)) / 2 * a; 
            cout << "x1=" << x1 << ",x2=" << x2 << endl;
        }
    }
    system("pause");
    return 0;
}