To factor the expression \(8a - 28\) using the greatest common factor (GCF), we first identify the GCF of the coefficients \(8\) and \(28\).
The GCF of \(8\) and \(28\) is \(4\).
Now, we can factor \(4\) out of the expression:
\[ 8a - 28 = 4(2a - 7) \]
So, the factored form of \(8a - 28\) is:
\[ 4(2a - 7) \]