A rectangular vegetable field with a width of 3.5m is surrounded by a fence with a length of 16m and one wall. What is the maximum area of it?
用长16m篱笆靠一面墙围成一个宽3.5m的长方形菜地，它的面积最大是多少平方米？
<gadget id="calculator">3.5 * 2</gadget>
<output>7</output>

<gadget id="calculator">16 - 7</gadget>
<output>9</output>

<gadget id="calculator">9 * 3.5</gadget>
<output>31.5</output>

<result>31.5</result>