Terribly need help trying to simplify this equation

Hi guys! first post here and i have been trying to figure this one out for quite some time no and i really cant seem to crack it no matter where i look at it, which is why i turn to you guys, the only people who can help me. Actually i just dont follow the sequence as the answer was already shown, i just dont understand the transition:

= x^3 + y^3 + x^2y + xy^2

= (x+y)^3 - 3xy(x+y) + xy(x+y)

I really need you guys' help to understand this one, ive been at this for at least a good hour and still no luck. please help!