First, find the prime factorization of both numbers.
378/2 = 189
189/3 = 63
63/3 = 21
21/3 = 7
7/7 1
378 = 2 * 3^3 * 7
132/2 = 66
66/2 = 33
33/3 = 11
11/11 = 1
132 = 2^2 * 3 * 11
For the GCF, choose the common factors with lower exponent.
378 = 2 * 3^3 * 7
132 = 2^2 * 3 * 11
From 2 and 2^2 choose 2.
From 3 and 3^2 choose 3.
7 and 11 are not common, so do not choose them.
GCF = 2 * 3 = 6