The first phase of the Nanjing Metro project is divided into two parts: the elevated line and the underground line. The elevated line is about 8.5 kilometers long, and the length of the underground line is 3.4 kilometers less than twice that of the elevated line. How much longer is the length of the underground line than the overhead line?
南京地铁一期工程分高架线和地下线两部分，其中高架线大约长8.5千米，地下线的长度比高架线的2倍少3.4千米．地下线的长度比高架线长多少千米？
<gadget id="calculator">8.5 * 2</gadget>
<output>17</output>

<gadget id="calculator">17 - 3.4 - 8.5</gadget>
<output>5.1</output>

<result>5.1</result>