To find the greatest common factor (GCM) for monomials, as you have in this case, you can write the complete factorization of each monomial and find the common factors. Then:
[tex]\begin{gathered} x^4=x\cdot x\cdot x\cdot x \\ x^7=x\cdot x\cdot x\cdot x\cdot x\cdot x\cdot x \\ x^9=x\cdot x\cdot x\cdot x\cdot x\cdot x\cdot x\cdot x\cdot x \end{gathered}[/tex]As you can see, all of these terms have x*x*x*x in common, and this is equal to x^4.
Thus, the GCF for this list of terms is:
[tex]x^4[/tex]