okay let me explain ... first of all do not use that word SIR .. just a kid like u ... may made it earlier to IIT thats it . ok

 

Now about the approach .. see the letter E can appear in a four letter word for one , two or three times.. OKay ?? Now if the letter E appears 3 times then we need to divide by factorial(3) since otherwise we are counting redundant case.

So note that the x^3 has been divided by factorial(3) which is 6.

Similarily for other letters. Try to understand that x^n denotes that the letter appears n times in the word. Now is it clear ???