A vegetable greenhouse covered with plastic film is 12 meters long and its cross-section is a semicircle with a radius of 3 meters. About how many square meters of plastic film will be used to build this greenhouse?
一个用塑料薄膜覆盖的蔬菜大棚，长12米，横截面是一个半径为3米的半圆．搭建这个大棚大约要用多少平方米的塑料薄膜？
<gadget id="calculator">3 ** 2</gadget>
<output>9</output>

<gadget id="calculator">3.14 * 9</gadget>
<output>28.26</output>

<gadget id="calculator">12 / 2</gadget>
<output>6</output>

<gadget id="calculator">2 * 3.14 * 3 * 6</gadget>
<output>113.04</output>

<gadget id="calculator">28.26 + 113.04</gadget>
<output>141.3</output>

<result>141.3</result>