Incorrect user input - c++

Question

Trying to let only numbers be a vaild input for a quadratic equation solver. I used bool and broke my code and no matter the input it just gives me my error message even if it is a vaild input.
After the progarm ask you to confirm the coefficients are correct, even if you put Y you get the " Please input a number. Please try again". how do you fix this?
I added the bool becasue that what my teacher showed me to use for checking inputs, very new to c++ and coding. I just added the whole code, the HW was to update HW 5 to do a few things more. I broke it trying to check inputs, when adding bool.
#include <iostream>
#include <cmath>
#include <string>
using namespace std;
int main()
{
string first, last;
double a, b, c, x1, x2, discriminant, realpart, imaginarypart;
char ch;
cout << "Please enter First and Last name," << endl;
cout << " " << endl;
cout << "First Name= ";
cin >> first;
cout << "and" << endl;
cout << "Last Name=";
cin >> last;
cout << " " << endl;
cout << " Hello " << first << " " << last << " "
<< "welcome." << endl;
cout << " " << endl;
bool isCorrect{ true };
do {
start:
isCorrect = true;
cout << "Enter the coefficients of a: ";
cin >> a;
cout << " " << endl;
cout << "Enter the coefficients of b: ";
cin >> b;
cout << " " << endl;
cout << "Enter the coefficient of c: ";
cin >> c;
cout << " " << endl;
cout << " A = " << a;
cout << " B = " << b;
cout << " C = " << c << endl
<< endl;
cout << " Confirm the coefficients value are correct or not (y/n): " << endl;
cout << " " << endl;
cin >> ch;
if (ch == 'n' || ch == 'N')
goto start;
cout << " " << endl;
discriminant = b * b - 4.0 * a * c;
if (cin.fail())
;
{
isCorrect = false;
cin.clear();
cin.ignore(245, '\n');
cout << " Please input a number. Please try again" << endl;
}
} while (!isCorrect);
bool isExit(false);
if (a == 0) {
cout << " " << endl;
cout << "Error message: a can not = zero (0) " << endl;
}
else if (discriminant > 0) {
x1 = (-b + sqrt(discriminant)) / (2.0 * a);
x2 = (-b - sqrt(discriminant)) / (2.0 * a);
cout << "Roots are real and different." << endl;
cout << " " << endl;
cout << "x1 = " << x1 << endl;
cout << "x2 =" << x2 << endl;
}
else if (discriminant == 0) {
cout << "Roots are real and same." << endl;
cout << " " << endl;
x1 = -b / (2.0 * a);
cout << "x1 = x2 ="
" "
<< x1 << endl;
}
else {
//cout << "Error message: No real solution exist." << endl; // Part 1 error code for no real solutions
realpart = -b / (2.0 * a); //Code for part 2
imaginarypart = sqrt(-discriminant) / (2.0 * a); // Code for part 2
cout << "Roots are complex and different." << endl; // Code for part 2
cout << " " << endl;
cout << "x1 = " << realpart << "+" << imaginarypart << "i" << endl; // Code for part 2
cout << "x2 = " << realpart << "-" << imaginarypart << "i" << endl; // Code for part 2
}
cout << " " << endl;
cout << " Would you like to solve another quadratic equation (y/n): " << endl;
cin >> ch;
if (ch == 'y' || ch == 'Y')
goto start;
else
(ch == 'n' || ch == 'N');
return 0;
}