Can someone explain to me how this works?

(x+y)^2+2(x+y)=(x+y)(x+y+2)

!

Well i think it works because the distributive property is used on the variables on the left side of course to get x^2+y^2+2x+2y, and then when you multiply (x+y) and (x+y+2) you get x^2+y^2+2x+2y which would be equal to the left side. 
anyway, that's just my interpretation. hope this helped!

The answer for you question is b