Explanation:

For x = 7, the expression becomes 72+7+17 = 73
which is a prime number.
For x = 11, the expression becomes 112+11+17 = 149
which is again a prime number.
For x = 13,the expression becomes 132+13+17 = 199 which is again a prime number.
For x = 17, the expression is surely a composite number because then 17 can be taken common from all the terms.
Hence it will be divisible by 17.
Hence the least value of x for which the expression is not a prime number is x = 17.