Write  with a negative exponent, 



Multiply exponents and write  as  



Multiply exponents again and write  as 



Write  as  and then as 



Next add the exponents of x:  and get 



Now remember what I said in the beginning.
To becom a constant the variable x must be raised to the 0 power:

So we set the exponent of x equal to 0





So we substitute 3 for x:









For your information so as to see what the entire expansion
looks like when simplified:

 

So the constant term is really the 4th term, when k=3.

Edwin