To factor the expression \(12x + 27\) completely, we start by looking for the greatest common factor (GCF) of the two terms.
The coefficients are 12 and 27. The GCF of 12 and 27 is 3.
Now we can factor out the GCF:
\[ 12x + 27 = 3(4x) + 3(9) = 3(4x + 9) \]
Thus, the completely factored form of the expression \(12x + 27\) is:
\[ \boxed{3(4x + 9)} \]