To prove this identity, we can start by expending the left side:

Now, we can apply the distributive property in two of these factors:

Now, we substitute it back:

And now, we can apply the distributive property again:

Going back to the equation, we have:

The two middle terms have the factors "3", "x" and "y" in common, so we can factor them out:

And we end up with the identity we wantd to prove, so it is proved.