private void substractButton_Click(object sender, EventArgs e)
{
InitializeVariable();
string result = aCalculator.Substract(firstNumber, secondNumber).ToString();
resultTxtBox.Text = result;
}
static void Main(string[] args)
{
Calculator calc = new Calculator();
double a = 4.5, b = 2.5;
double result = calc.Add(a, b);
Console.WriteLine($"result = {result}");
double result2 = calc.Substract(a, b);
result2 = calc.Substract(a, b);
Console.WriteLine($"result2 = {result2}");
double result3 = calc.Multiple(a, b);
result3 = calc.Multiple(a, b);
Console.WriteLine($"result3 = {result3}");
double result4 = calc.Divide(a, b);
result4 = calc.Divide(a, b);
Console.WriteLine($"result4 = {result4}");
}