一件夹克衫先按成本提高50%标价，再以8折（标价的80%）出售，结果获利28元．如果设夹克衫的成本是x元，据题意可列得方程为____．

探究杠杆的平衡条件：

【提出问题】如图1所示，是一种常见的杠秤，此时处于水平位置平衡．

发现一：小明在左侧挂钩上增加物体，可观察到提纽左侧下沉，他认为改变杠杆的水平平衡可以通过改变作用在杠杆上的____ 来实现；

发现二：接着小新移动秤砣使其恢复水平位置平衡，说明通过改变____ 的长短也可以改变杠杆的平衡．

那么，杠杆在满足什么条件时才平衡呢？

【制定计划与设计实验】

实验前，轻质杠杆处于如图2所示的状态，使用时，首先应将杠杆的平衡螺母向____ （选填“左”或“右”）调节，使杠杆处于水平位置平衡，这样做的好处是____ ．

【实验结论】

如图3所示，他们进行了三次实验，对实验数据进行分析，得出杠杆的平衡条件是　____ 　．

【拓展应用】

如图4所示，是用手托起重物的示意图，图中前臂可以看作是一个____ 杠杆（选填“省力”、“费力”或“等臂”）．假如托起6N的重物，请你根据图4所示，估算出手臂要用的动力大约是　____ 　N．

瓦子在宋代城市的盛行，主要是因为（   ）

   Do you still remember the disaster(灾难)several years ago? In Indonesia (印度尼西亚) thousands of people were killed by the tsunami(海啸) that washed people out to sea and pulled children out of their parents’ arms. “The weather was fine with no clouds .There was no warning and suddenly the sea water just hit the city. In some parts the water was as high as two meters.” a man in the city said. “Sadli, an official at the hospital, said many of the dead in the city were children under the age of ten.” “Maybe the children were being carried by their hold but they fell over in the water and couldn’t hold on to their children .All the dead children drawed.” said Sadli. Officials said hundreds of houses had been swept away the force (力量) of five –meter –high waves across Aceh, which lies on the northern tip (末端)of Sumatra Island . “Tens of people saw the rushing water and didn’t run to high ground .Instead, they kept watching it .Several of my workmates are missing.”  Lelana said by telephone.

如果向东走3米记作+3米，那么向西走2米记作（   ）