[dart/difference-of-squares] Create mentoring.md

tracks/dart/exercises/difference-of-squares/mentoring.md

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+# Mentoring
+
+## Reasonable solutions
+
+We need to have two three methods:
+
+- the first one is sqareOfSum that will return the sum of all numbers and squared afterwards
+- the second method is the sumOfSquares which returns the sum of sqared numbers
+- the last method - differenceOfSquares - returns the difference of both of the methods above
+
+This exercise allows students to be tidy in coding their solution. Putting everything in methods that can be used in other methods is welcomed practice.
+
+```dart
+class DifferenceOfSquares {
+  int squareOfSum(int n) => pow(List<int>.generate(n, (i) => i + 1).reduce((a,b) => a+b), 2);
+  int sumOfSquares(int n) => List<int>.generate(n, (i) => pow(i+1, 2)).reduce((a,b) => a+b);
+  int differenceOfSquares(int n) => squareOfSum(n) - sumOfSquares(n);
+}
+```
+
+
+## Common suggestions
+
+- they need to create three methods
+- use [generate-method][reference-generate-method] to generate a List of incremented numbers
+- [reduce-method][reference-reduce-method] sums the generated list 
+- [pow-method][reference-pow-method] is a maths function to put x to the power of y
+
+[reference-generate-method]: https://fluttermaster.com/how-to-use-list-generate-in-dart/
+[reference-reduce-method]: https://api.dart.dev/stable/2.18.1/dart-core/Iterable/reduce.html
+[reference-pow-method]: https://api.dart.dev/stable/2.10.5/dart-math/pow.html
+
+
+## Solution Video
+
+Step by step [video][youtube-video] guide solution for this exercise
+
+[youtube-video]: https://www.youtube.com/watch?v=iwvcEKG_2qU