Use two triangular pieces of paper with three sides of 3 cm, 5 cm and 7 cm to form a parallelogram. What is the longest perimeter of the assembled parallelogram in centimeters?
用两个三条边分别是3厘米、5厘米和7厘米的三角形纸片拼成一个平行四边形，拼成的平行四边形的周长最长是多少厘米．
<gadget id="calculator">7 + 5</gadget>
<output>12</output>

<gadget id="calculator">12 * 2</gadget>
<output>24</output>

<result>24</result>