If a polynomial function f (x) has roots 3 and / sqrt (7), what must also be a root of f (x) ?

When one of the roots of a polynomial function is an irrational number that cannot be expressed in any other way possible, it is known that its conjugate must also be a root of the function. If 3 (sqrt of 7) is a root then, - 3 (sqrt of 7) is also a root.