\$ n \$ is correctly represented by \$ x ^ 2 + xy + y ^ 2 \$ if and only if \$ n \$ is not divisible by 9 or any premium 3k + 2 \$

How can I show a positive integer $$n$$ is correctly represented by the quadratic binary form $$x ^ 2 + xy + y ^ 2$$ if and only if $$n$$ is not divisible by 9 or any prime number of the form $$3k + 2$$?

I'm going through Niven chapters 3.6 to 3.7, Introduction to Number Theory.