In polynomials, there's a solved example of the problem 

(2 x  + 5 y ) 3  − (2 x  − 5 y ) 3

I have not understood the alternative (shorter) method. Could you please explain each step in detail? The link of the problem is given below: 

http://cbse.meritnation.com/cbse-math-math/polynomials/7/99/studymaterials.html

We will be able to offer you meaningful help only if you can clearly write down the specific problem areas/ topics in a given lesson. We look forward to hear from you again!

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