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$this->request()->get('req3') is used to get the third value in the url. For example: http://\{YourSite.com\}/index.php/req1/req2/req3/...
In this case, current url is: http://YourSite.com/index.php/to-do-list/view/todo-id/todo-title, so
request()->get('req3')will return the id of the Todo List
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