SeperateCoefficients & Gauss Seidel iteration

#include <bits/stdc++.h>

using namespace std;

void seperateCoefficient(string eqn, int a[])
{
    bool isNegative = false;
    int i, sum = 0;

    //ax + by + cz
    for(i = 0; eqn[i] != '='; i++){
        if(eqn[i] == '-')
            isNegative = true;
        else if(eqn[i] == '+')
            isNegative = false;
        else if(eqn[i] >= '0' && eqn[i] <= '9'){
            sum *= 10;
            sum += eqn[i] - '0';
        }
        else if(eqn[i] == 'x' || eqn[i] == 'X'){
            if(isNegative)
                sum = -sum;
            a[0] = sum;
            sum = 0;
        }
        else if(eqn[i] == 'y' || eqn[i] == 'Y'){
            if(isNegative)
                sum = -sum;
            a[1] = sum;
            sum = 0;
        }
        else if(eqn[i] == 'z' || eqn[i] == 'Z'){
            if(isNegative)
                sum = -sum;
            a[2] = sum;
            sum = 0;
        }
    }
    // = const
    isNegative = false;
    for(; i < eqn.length(); i++){
        if(eqn[i] == '-')
            isNegative = true;
        else if(eqn[i] >= '0' && eqn[i] <= '9'){
            sum *= 10;
            sum += eqn[i] - '0';
        }
    }
    if(isNegative)
        sum = -sum;
    a[3] = sum;
}


int main()
{
    int a[3][4];
    string eqn;

    for(int i = 0; i < 3; i++){
        getline(cin, eqn);
        seperateCoefficient(eqn, a[i]);
    }

    //printing
    cout << endl << "Co-efficients: " << endl;
    for(int i = 0; i < 3; i++){
        for(int j = 0; j < 4; j++){
            cout << a[i][j] << "  ";
        }
        cout << endl;
    }

    double x = 0, y = 0, z = 0;
    for(int i = 0; i < 20; i++){
        x = (a[0][3] - a[0][1]*y - a[0][2]*z) / a[0][0];
        y = (a[1][3] - a[1][0]*x - a[1][2]*z) / a[1][1];
        z = (a[2][3] - a[2][0]*x - a[2][1]*y) / a[2][2];
    }
    cout << endl << "Solution:" << endl;
    cout << "x = " << x << endl;
    cout << "y = " << y << endl;
    cout << "z = " << z << endl;
    return 0;
}