If you know that 7 - square root 5 is it root of a polynomial function

As you must know, If one root of the polynomial 7-√5, the other will be 7+√5 i. e irrational root occur in pairs.

A polynomial function cannot have single unreal i. e irrational root. It always occur in pairs.

So, consider a polynomial function of any degree, if it has a root 7-√5, then it must have another root as 7+√5.

A polynomial can't have 7-√5 as a single root.