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