The first phase of the Nanjing Metro project is divided into two parts: the elevated line and the underground line. Among them, the length of the elevated line is about 6.5 kilometers, and the length of the underground line is 0.21 kilometers longer than that of the elevated line by 1.4 times. How many kilometers is the total length of the underground line?
南京地铁一期工程分高架线和地下线两部分．其中高架线长约6.5千米，地下线的长度比高架线的1.4倍还多0.21千米．地下线全长约多少千米？
<gadget id="calculator">6.5 * 1.4</gadget>
<output>9.1</output>

<gadget id="calculator">9.1 + 0.21</gadget>
<output>9.31</output>

<result>9.31</result>