63x^6 = 3 * 3 * 7 * x * x * x * x * x * x
42x^2 = 2 * 3 * 7 * x * x
21x^5 = 3 * 7 * x * x * x * x * x
The GCF is made of the factors that appear in 63x^6, 42x^2, and 21x^5. In this case that's a 3, a 7, and two x's. That makes 3 * 7 * x * x = 21x^2
Answer: The GCF of 63x^6, 42x^2, and 21x^5 is 21x^2.