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