Convert String that contains a math expression with multiple operands to an int?

Question

I am creating this mini program that gives basic math problems to the user, and the user has to answer the questions. I am doing this by creating an array with basic math problems, contains about 20, and the way the user inputs the answer is through a Scanner.

My problems is I have searched and found a way to convert a string with only 1 operator and 2 operands into an int, but I have been trying to modify it so the program can convert the strings that have multiple parts into an int, but I cant. This is what I have

Problems from array:

private String arithProb[] = {
    "40 + 20",
    "60 - 30 + 60",
    "2 * 7 + 40 - 20 + 20 -40"

};

These are just 3 of the 20. The first one outputs the correct answer, which is 60 but the others output the result from the first 2 operands and first operator. second problem only outputs 30. third problem only outputs 14.

Here is the code I am using to convert the string into an int. What am I doing incorrectly or what am I not doing to not get the full thing working?

public static void solveBasicArithmetic(String prob, int ans){
    int cAns = evaluateQuestion(prob);

    System.out.println();
    if(ans == cAns){
        System.out.println("CORRECT!");
    }else if(ans != cAns){
        System.out.println("Oops, the correct answer is: " + cAns);
    }
    System.out.println();

}

public static int evaluateQuestion(String problem){
    Scanner sc = new Scanner(problem);
    int finalAns;

    do{
        //Get the next number from the Scanner
        int firstValue = Integer.parseInt(sc.findInLine("[0-9]*"));

        //Get everything which follows and is not a number (might contain white 
        spaces)

        String operator = sc.findInLine("[^0-9]*").trim();
        int secondValue = Integer.parseInt(sc.findInLine("[0-9]*"));

        switch(operator){
            case "+": 
                finalAns =+ firstValue + secondValue;
                return finalAns;

            case "-":
                finalAns =+ firstValue - secondValue;
                return finalAns;

            case "/": 
                finalAns =+ firstValue / secondValue;
                return finalAns;

            case "*": 
                finalAns =+ firstValue * secondValue;
                return finalAns;

            case "%": 
                finalAns =+ firstValue % secondValue;
                return finalAns;

            default: throw new RuntimeException("Unknown operator: " + operator);

        }

    }while(sc.findInLine("[0-9]*") != null);

}