To factor the expression \(-21x + 28\) completely, we can look for the greatest common factor (GCF) of the terms.
The GCF of \(-21\) and \(28\) is \(-7\).
Now, we can factor out \(-7\) from the expression:
\[ -21x + 28 = -7(3x - 4) \]
Thus, the completely factored form of the expression is:
\(-7(3x - 4)\)
So the correct response is:
−7(3x−4)