Something like this could work:
$numbers = [
1 => 'first',
2 => 'second',
3 => 'third',
4 => 'fourth'
];
$days = [
1 => 'monday',
2 => 'tuesday',
3 => 'wednesday',
4 => 'thursday',
5 => 'friday',
6 => 'saturday',
7 => 'sunday',
];
$months = [
1 => 'January',
2 => 'February',
3 => 'March',
4 => 'April',
5 => 'May',
6 => 'June',
7 => 'July',
8 => 'August',
9 => 'September',
10 => 'October',
11 => 'November',
12 => 'December'
];
$dateString = $numbers[$weekdayAmountSelect] .' '. $days[$weekdaySelect] .' of '. $months[$amount_of_months_option_2];
$date = \Carbon\Carbon::parse($dateString);
dd($date->toDayDateTimeString());
Probably best to change your inputs a bit so that you dont have to do the reformatting of 1 => monday, etc. just have the values of the selectbox have 'first / second / third' etc. and the actual english day/month names and pass it on to Carbon::parse();