Find the surface area of a cuboid: the length is 21 decimeters, the width and height are both 12 decimeters.
求长方体的表面积：长是21分米，宽和高都是12分米．
<gadget id="calculator">21 * 12</gadget>
<output>252</output>

<gadget id="calculator">12 * 12</gadget>
<output>144</output>

<gadget id="calculator">252 + 252 + 144</gadget>
<output>648</output>

<gadget id="calculator">648 * 2</gadget>
<output>1_296</output>

<result>1_296</result>