Question: Gcd of polynomials with scalars

How do I compute the gcd of polynomials with scalars e.g. how to compute gcd (x^9-a, bx^6-c )  as polynomials in x.   https://www.maplesoft.com/support/help/Maple/view.aspx?path=Task/GCDofPolynomials   here I don't see how to specify the polynomials are in x.

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