If you put x=y in the given equation it would make no change to the equation as y will take place of x only, Thus what you get will be
which is same as your given equation.
For finding a solution to any question the logic is to convert the question into general (predefined forms) which we have already learned.
The asked limit looks alot similar to the formula :
So, for converting the above format in this predefined format we put x+1=y
So we can write,
Here a=1, n=5 so the solution is