Final answer:
The greatest common factor of the given monomials is (g²h²).
Step-by-step explanation:
To find the greatest common factor of the given monomials (9g²h³, 6g⁵h², 21g⁴h²), we need to determine the highest power of each variable that appears in all three of them.
- For the variable 'g', the highest power that appears in all three monomials is g².
- For the variable 'h', the highest power that appears in all three monomials is h².
Therefore, the greatest common factor of the monomials is (g²h²).