Solve the equation: 3x-7=x+1.
解方程：3x-7=x+1．
<gadget id="calculator">7 + 1</gadget>
<output>8</output>

<gadget id="calculator">3 - 1</gadget>
<output>2</output>

<gadget id="calculator">8 / 2</gadget>
<output>4</output>

<result>4</result>