I am having difficulty understanding how my book came up with this answer.
Define $a \star b =ab+2b$, and suppose $x \star y = y \star x$. Then which of the following must be true?
A. $x+y=1$.
B. $y=0$.
C. $x=y$.
D. $x=-2$.
E. $xy=0$.
How did the text conclude that $x=y$ or C is the answer ?