If x = 2 , we have x2 + 2x = 8 ( scrap because 8 isn't a prime number)
If x = 3 , we have x2 + 2x = 17 ( satisfy )
If x > 3 and x is a prime number then x =3k + 1 or 3k + 2 ( k is a natural number)
+) x = 3k +1
If x = 3k +1 then x2 divide to 3 has the remainder 1
and 2x divide to 3 has the remainder 2
=> 2x + x2 divided to 3
But 2x + x2 > 3
So 2x + x2 isn't a prime number
Do the similar to x = 3k+2
The answer is x = 3
