The HCF is: 12
The GCF is 8.
(9r - 8)(9r + 8)
(9x + 8)(9x - 8)
(3t + 1)(t + 5)
Factor it. (x - 8)(x - 2) x = 2, 8
This is a difference of two square, so you can apply the factoring rule for the difference of two squares.
Sixteen factor pairs.
The greatest common factor (GCF) for twenty-one and seventy-two is three.