The upper base of the trapezoid is 3 cm long, the lower base is 5 cm long, and the two waists are 4 cm long and 7 cm long respectively. If two such trapezoids are assembled into a parallelogram, try to calculate its perimeter. (There are two cases)
梯形的上底长3厘米，下底长5厘米，两条腰长分别长4厘米、7厘米，如果把两个这样的梯形拼成一个平行四边形，试列式计算它的周长．(有两种情况)
<gadget id="calculator">3 + 5 + 4 + 7</gadget>
<output>19</output>

<gadget id="calculator">19 * 2</gadget>
<output>38</output>

<gadget id="calculator">7 * 2</gadget>
<output>14</output>

<gadget id="calculator">38 - 14</gadget>
<output>24</output>

<result>24</result>