Let \( a \) be an integer such that \( \displaystyle \lim_{x \to 7} \frac{18 - [ 1 - x ]}{[x - 3a]} \) exits, where \( [ t ] \) is greatest integer \( \le t \). Then \( a \) is equal to:
\( -6 \)
\( -2 \)
\( 2 \)
\( 6 \)