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