How to assign lambda expression to a variable?

I have a kind of calculator:

sum = value1 + value2;
    multiplication = value1 * value2;
    division = (value1 / value2);
    average = (sum / 2);

    if (operation.compareToIgnoreCase("add") == 0)
        result = sum;
    else if (operation.compareToIgnoreCase("mult") == 0)
        result = multiplication;
    else if (operation.compareToIgnoreCase("div") == 0)
        result = division;
    else if (operation.compareToIgnoreCase("avg") == 0)
        result = average;

And i have to turn this to lambda expressions:

result = (request.getValue1(), request.getValue2()) -> {
        if (request.getOperation().compareToIgnoreCase("add") == 0) return request.getValue1() + request.getValue2();
        else if (request.getOperation().compareToIgnoreCase("mult") == 0) return request.getValue1() * request.getValue2();
        else if (request.getOperation().compareToIgnoreCase("div") == 0) return request.getValue1() / request.getValue2();
        else if (request.getOperation().compareToIgnoreCase("avg") == 0) return (request.getValue1() + request.getValue2())/2;
    }

But it doesnt work. Can you guys help? Thanks