URGENT HELP How to add "+,-,x, / and =" to my 3D Calculator

Hi I'm making a 3d calculator that you can jump on each individual number for it to show up on the screen. but it only shows one number at a time and doesn't do the calculators basic functions such as addition, subtraction and also working out the total could someone help me on what i need to add to get this to work. Script i have so far:

 public float firstNumber = 0f, secondNumber = 0f;
 float product = 0f;
 public TextMeshProUGUI text;
 string op;
   
 public void Value(float value)
 {
     if (op == null)//** Let's us continue
     {
         firstNumber = value;
         text.text = firstNumber.ToString() + " ";
     }
     else
     {
         secondNumber = value;
         text.text += secondNumber.ToString() + " ";
     }
 }
 //this means the script needs to be on the player
 void OnTriggerEnter(Collider other)
 {
     if ( other.gameObject.CompareTag("button"))
     {
         
        
         Debug.Log(firstNumber.ToString());
         Value(float.Parse(other.gameObject.name));
     }
 }
 public void Operation(string operation)
 {
     if (operation == "+")
     {
         op = "+ ";
     }
     else if (operation == "-")
     {
         op = "- ";
     }
     else if (operation == "=")
     {
         Calculate();
     }
     text.text += op;
 }



 void Calculate()
 {
     if (op == "+ ")
     {
         product = firstNumber + secondNumber;
     }
     else if (op == "- ")
     {
         product = firstNumber - secondNumber;
     }
     text.text += "= " + product.ToString();

     //Resets
     op = null;
     firstNumber = product;
     secondNumber = 0;
 }

}