My solution

app/Http/Controllers/CountryController.php

@@ -9,7 +9,7 @@ class CountryController extends Controller
     public function index()
     {
         // TASK: load the relationship average of team size
-        $countries = Country::all();
+        $countries = Country::withAvg('teams', 'size')->get();
 
         return view('countries.index', compact('countries'));
     }

app/Http/Controllers/ProjectController.php

@@ -10,7 +10,8 @@ public function store(Request $request)
     {
         // TASK: Add one sentence to save the project to the logged-in user
         //   by $request->project_id and with $request->start_date parameter
-
+        $request->user()->projects()->attach($request->project_id, ['start_date' => $request->start_date]);
+
         return 'Success';
     }
 }

app/Http/Controllers/UserController.php

@@ -8,8 +8,8 @@ class UserController extends Controller
 {
     public function index()
     {
-        $users = User::all();
-
+        $users = User::has('projects')->get();
+        
         return view('users.index', compact('users'));
     }
 

app/Models/Attachment.php

@@ -14,5 +14,6 @@ class Attachment extends Model
     public function attachable()
     {
         // TASK: fill in the code to make it work
+        return $this->morphTo();
     }
 }

app/Models/Role.php

@@ -14,6 +14,6 @@ class Role extends Model
     public function users()
     {
         // TASK: fix this by adding a parameter
-        return $this->belongsToMany(User::class);
+        return $this->belongsToMany(User::class, 'users_roles', 'role_id', 'user_id');
     }
 }

app/Models/Task.php

@@ -13,6 +13,6 @@ class Task extends Model
 
     public function user()
     {
-        return $this->belongsTo(User::class, 'users_id');
+        return $this->belongsTo(User::class, 'users_id')->withDefault();
     }
 }

app/Models/Team.php

@@ -14,7 +14,7 @@ class Team extends Model
     public function users()
     {
         // TASK: fix this by adding some extra code
-        return $this->belongsToMany(User::class);
+        return $this->belongsToMany(User::class)->withPivot('position', 'created_at');
     }
 
 }

app/Models/User.php

@@ -45,12 +45,13 @@ class User extends Authenticatable
     public function tasks()
     {
         // TASK: fix this by adding a parameter
-        return $this->hasMany(Task::class);
+        return $this->hasMany(Task::class, 'name');
     }
 
     public function comments()
     {
         // TASK: add the code here for two-level relationship
+        return $this->hasManyThrough(Comment::class, Task::class, 'name', 'task_id');
     }
 
     public function projects()