To factor the expression \(8a - 28\) using the greatest common factor (GCF), we first need to find the GCF of the coefficients 8 and 28.
The factors of 8 are: \(1, 2, 4, 8\)
The factors of 28 are: \(1, 2, 4, 7, 14, 28\)
The greatest common factor of 8 and 28 is 4.
Now, we can factor out the GCF from the expression \(8a - 28\):
\[ 8a - 28 = 4(2a - 7) \]
Therefore, the factored form of \(8a - 28\) is
\[ \boxed{4(2a - 7)} \]