"Expecting member declaration"

I'm having trouble with these lines of code:

class MainActivity : ComponentActivity() {
    var item = "None"
    var price: Double = 16.99
    var Quantity: Int = 2
    println(Calculate(price, item, Quantity)) 
}
fun Calculate(price: Double, item: String, Quantity: Int): Double {
    return(Quantity*price)
}

You can't have statements directly under a class, they should be inside a constructor or a method. E.g.:

class MainActivity  {
    var item = "None"
    var price: Double = 16.99
    var Quantity: Int = 2
    fun someFunc() {
        println(Calculate(price, item, Quantity)) 
    }
}