I dont understand how to complete this challenge??

I dont understand how to do this challenge I know that I know how to do it but I just can't figure out what I am missing. PLEASE HELP?? I need this to move on since this is a. Ending challenge.

DIRECTIONS I've provided a base class Person in the editor below. Once an instance of Person is created, you can call getFullName() and get a person's full name.

Your job is to create a class named Doctor that overrides the getFullName() method. Once you have a class definition, create an instance and assign it to a constant named someDoctor.

For example, given the first name "Sam", and last name "Smith", calling getFullName() on an instance of Person would return "Sam Smith", but calling the same method on an instance of Doctor would return "Dr. Smith".

class Person { let firstName: String var lastName: String

init(firstName: String, lastName: String) {
    self.firstName = firstName
    self.lastName = lastName
}

func getFullName() -> String {
    return "\(firstName) \(lastName)"
}

}

// Enter your code below

class Doctor: Person {
    override func getFullName() -> String {
        return "Dr. \(lastName)"
    }
}
let someDoctor = Doctor.getFullName("Smith")

Sorry for the mix match format I dont know why it does that but please try to read and help me. Much appreciated.

class Person {
    let firstName: String
    let lastName: String

    init(firstName: String, lastName: String) {
        self.firstName = firstName
        self.lastName = lastName
    }

    func getFullName() -> String {
        return "\(firstName) \(lastName)"
    }
}

// Enter your code below