Eloquent Puzzles 03: SUM with Relationship Conditions

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Question #1 Question from Laracasts forum, let's try to solve it.

Look at the "Code snippet" section - user can have many bookings and each booking has many different items.
I want to get a user's sum of "item_num" column for a particular user but only of "confirmed" bookings.

Please write that Eloquent query. Something like:

$sum = BookingItem:: [... where user ID is 1 ...] AND [ ... where booking status is confirmed ... ] -> [GET SUM OF 'item_num'];
Code snippet
Booking.php 
Columns: id, status [confirmed, pending], user_id, timestamps

public function user() {
   return $this->belongTo(User::class);
}

//------------------------------------------------------------

BookingItem.php
Columns: id, booking_id, item_type, item_num, timestamps

public function booking() {
   return $this->belongTo(Booking::class);
}
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