Given that the polynomial x ^ 2 + ax-9 can be factorized in the range of integers, then the value of natural number a is 8, why should - 8 be abandoned?

Given that the polynomial x ^ 2 + ax-9 can be factorized in the range of integers, then the value of natural number a is 8, why should - 8 be abandoned?

Natural number is used to measure the number of things or to indicate the order of things Includes positive integers and 0
-8 is an integer but not a natural number