To factor the expression \(-21x + 28\), we can find the greatest common factor (GCF) of the coefficients -21 and 28.
The GCF of -21 and 28 is 7. We can factor out -7 as follows:
\[ -21x + 28 = -7(3x - 4) \]
Thus, the expression factored completely is:
\[ -7(3x - 4) \]