Quadratic Equation Program Assistance

[kymattem]

For my next C++ class assignment, I need to write a program which can give the value of "x" in a quadratic equation when inputting the values of a, b, and c. I've written up a program which largely accounts for all the scenarios, and which checks out in the compiler. However, when I go to run it, and I put in values for a, b, and c, the program seems to skip back to the input section. Does anyone know what I did wrong in this?

#include <fstream>
#include <iomanip>
#include <iostream>
#include <math.h>
#include <stdlib.h>

using namespace std;

int main()
{
    ofstream output ("c:result.dat");
    int a,b,c;  
    double d,x1,x2;
    while(a+b+c>0)
    {
cout<<"Enter the value of a, b and c. ";
output<<"Enter the value of a, b and c. ";
cin>>a>>b>>c;
d=pow(b,2)-4*a*c;
if((a+b+c)<=0)
{
break;
}
if(a=0)
{
       if(b=0)
       {
       cout<<"This is not an equation."<<endl;
       output<<"This is not an equation"<<endl;
       }
       else
       {
       cout<<"The equation is linear. x = "<<(-c/b)<<endl;
       output<<"The equation is linear. x = "<<(-c/b)<<endl;
       }
}
if(d>0)
{
x1=(-b/(2*a))+(sqrt(d)/(2*a));
x2=(-b/(2*a))-(sqrt(d)/(2*a));
       cout<<"x1 = "<<x1<<endl;
       cout<<"x2 = "<<x2<<endl;
       output<<"x1 = "<<x1<<endl;
       output<<"x2 = "<<x2<<endl;
}
else if(d=0)
{
     cout<<"x = "<<(-b/(2*a))<<endl;
     output<<"x = "<<(-b/(2*a))<<endl;
     }
else if(d<0)
{
cout<<"The answers are complex numbers."<<endl;
cout<<"x1 = "<<(-b/(2*a))<<" + "<<(sqrt(-d)/(2*a))<<"i"<<endl;
cout<<"x2 = "<<(-b/(2*a))<<" - "<<(sqrt(-d)/(2*a))<<"i"<<endl;
output<<"The answers are complex numbers."<<endl;
output<<"x1 = "<<(-b/(2*a))<<" + "<<(sqrt(-d)/(2*a))<<"i"<<endl;
output<<"x2 = "<<(-b/(2*a))<<" - "<<(sqrt(-d)/(2*a))<<"i"<<endl;
}
}
return 0;
}

[norm b]

Hint: A single "=" is the assignment operator.

[anup30]

#include <iostream>
#include <cmath>
using namespace std;

int main()
{
	double a = 10 , b= -20 , c = -30;    
    //ax^2 + bx +c = 0    
    
    double x1 = (-1*b + sqrt(b*b - 4*a*c) ) /(2*a) ;
    double x2 = (-1*b - sqrt(b*b - 4*a*c) ) /(2*a) ;
    
    cout << x1 << " " << x2; // 3 -1
	    
return 0;    
}