-7≤[2(1+3x)]/7≤9
即-7*7/2 ≤ (1+3x)≤9*7/2
-49/2 ≤ (1+3x)≤ 63/2
-49/2 - 1 ≤ 3x≤ 63/2 -1
-51/2≤ 3x≤ 61/2
∴ -17/2≤ x≤ 61/6