The Conjugate Root Theorem states that if a polynomial has rational coefficients and a complex number is a root of the polynomial (where and are real numbers and ), then its complex conjugate is also a root of the polynomial.
In this case, since the quartic polynomial has rational coefficients and is a root. According to the Conjugate Root Theorem, must also be a root.
Now, for the root . If a polynomial with rational coefficients has a root of the form , then is also a root. Here