How do I update repeated answer ,button disable in this chapter

hey as per the chapter 3 challenge where we have to disable the button when it answered I have implemented it into the view model but it’s not working correctly when I press answer its disabled but when I change and come back its not

my codes
viewmodel code

var currentQuestionAnswered:Boolean=false
        get()=questionBank[currentIndex].answered

fun isAnswered() code

if (quizViewModel.currentQuestionAnswered==true) {
            trueButton.isEnabled = false
            falseButton.isEnabled = false
        } else {
            trueButton.isEnabled = true
            falseButton.isEnabled = true

        }