Replacing a with -2:

3(-2) + 6 + 12(-2) - 4 + 6(-2) - 12
= -6 + 6 - 24 - 4 - 12 - 12
= -30

Therefore, the expression 3a + 6 + 12a - 4 + 6a - 12, when a = -2, equals -30.