A parallelogram-shaped vegetable field, one side is 9 meters long and the other side is 3 meters shorter, how long is the fence needed to surround this vegetable field?
一块平行四边形的菜地，一条边长9米，比另一条边短3米，围这块菜地需要多长栅栏？
<gadget id="calculator">9 + 9 + 3</gadget>
<output>21</output>

<gadget id="calculator">21 * 2</gadget>
<output>42</output>

<result>42</result>