How to get the created_at in pivot table in laravel

Im confused how to get the created_at column in my pivot table. I have exam_user table with created_at and updated_at column. Now, I have this code in my view:

@if($exam->users()->where('user_id', Auth::user()->id)->exists())
    {{ Auth::user()->assignments()->pivot->created_at }}
    <div class="alert alert-success">You have done with this exam.</div>
@else

Any idea for this?