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