Alternatively:
Recall Pascal's Triangle:
1 1
1 2 1
1 3 3 1
1 4 6 4 1
1 5 10 10 5 1
...in which elements in a row are the sum of the two elements in the row above it. These numbers are the
Binomial Coefficients.

is the cube of the binomial

so the coefficients can be taken from the third row above:
1 3 3 1
and the exponents on

start with

and count down, while the exponents on

start with

and count up:

which simplifies to:
as previously.