Missing number is 6. All opposite numbers add up to 21.

缺少的数字是6，每组相对数字的和为21。 。

Both Numbers may slide synchronously in opposite directions.

两数可以同步地向相反方向移位。

When the direction is judged the feedback - sector Numbers must be identified. When the minus feedback is formed, the factors in the opposite direction should be odd number.

在化工控制回路中判断调节器作用方向时，可根据回路中负反馈环节个数来确定，当形成回路负反馈时，反作用方向的环节数应为单数个。