error: Conditions must have a static type of 'bool'

I tried to make a calculator in flutter for the first time, but an error occur at the bool type that i don't understand. (on the "if" opertaion)

class _CalculatorAppState extends State<CalculatorApp> {
  int? firstNum;
  int? secondnum;
  String? textToDisplay;
  String? history = '';
  String? res = '';
  String? operation;

  void btnOnClick(String btnVal){
    print(btnVal);
    if( btnVal == 'C'){
      textToDisplay = '';
      firstNum = 0;
      secondnum = 0;
      res= '';
    } else if (btnVal == 'C'){
      textToDisplay = '';
      firstNum = 0;
      secondnum = 0;
      res= '';
      history = '';
    } else if ( btnVal == '+'||
        btnVal == '-'||
        btnVal == '/'||
        btnVal == 'X') {
      firstNum = int.parse(textToDisplay!);
      res = '';
      operation = btnVal;
    } else if (btnVal == '='){
      secondnum = int.parse(textToDisplay!);
      if(operation = '+') {
        res = (firstNum! + secondnum!).toString();
        history = firstNum.toString() + operation.toString() + secondnum.toString();
      }
      if(operation = '-') {
        res = (firstNum! - secondnum!).toString();
        history = firstNum.toString() + operation.toString() + secondnum.toString();
      }
      if(operation = 'X') {
        res = (firstNum! * secondnum!).toString();
        history = firstNum.toString() + operation.toString() + secondnum.toString();
      }
      if(operation = '/') {
        res = (firstNum! / secondnum!).toString();
        history = firstNum.toString() + operation.toString() + secondnum.toString();
      } else {
        res = int.parse(textToDisplay! + btnVal).toString();
      }

      setState(() {
        textToDisplay = res;
      });
    }
  }

error: Conditions must have a static type of 'bool'. (non_bool_condition at [calculator] lib\main.dart:46)

error: Conditions must have a static type of 'bool'. (non_bool_condition at [calculator] lib\main.dart:50)

error: Conditions must have a static type of 'bool'. (non_bool_condition at [calculator] lib\main.dart:54)

error: Conditions must have a static type of 'bool'. (non_bool_condition at [calculator] lib\main.dart:58)

You're currently typing operation = 'x' which isnt valid syntax. You need a double '=', so:

if (operation == 'x') {
  // do stuff
}