已知a=(3,1),b=(x,-2),且a⊥b,则x的值为( )
①
<img src="20190823/1566540550755497.png" title="1566540550755497.png" alt="图片6.png" width="17" height="45"/>
②
<img src="20190823/1566540571684399.png" title="1566540571684399.png" alt="图片8.png" width="26" height="45"/>
③
<img src="20190823/1566540562849143.png" title="1566540562849143.png" alt="图片7.png" width="18" height="45"/>
④
<img src="20190823/1566540579921674.png" title="1566540579921674.png" alt="图片9.png" width="26" height="45"/>