For the numbers a, b, c, d, stipulate, <a, b, c, d>=2ab-c+d, given <1, 3, 5, x>=7, find the value of x.
对于数a、b、c、d，规定，＜a、b、c、d＞=2ab-c+d，已知＜1、3、5、x＞=7，求x的值．
<gadget id="calculator">7 - 1</gadget>
<output>6</output>

<result>6</result>