else if (isNumber3 == true && numericValue3 == 4)
{
    double c = a / b;
    Console.WriteLine($"The result of division of the two equals {c}.");
    valid3 = true;
}

else if (isNumber3 == true && numericValue3 == 4)
{
    double c = a / b;
    Console.WriteLine($"The result of division of the two equals {c}.");
    valid3 = true;
}