Given that △-1.7=2.2, △+□=6.7, then □=how many.
已知△-1.7=2.2，△+□=6.7，则□=多少．
<gadget id="calculator">2.2 + 1.7</gadget>
<output>3.9</output>

<gadget id="calculator">6.7 - 3.9</gadget>
<output>2.8</output>

<result>2.8</result>