Solve the equation: x-(5/21)=(8/21).
解方程：x-(5/21)=(8/21)．
<gadget id="calculator">5 / 21</gadget>
<output>5/21 = around 0.238095</output>

<gadget id="calculator">8 / 21</gadget>
<output>8/21 = around 0.380952</output>

<gadget id="calculator">(5/21) + (8/21)</gadget>
<output>13/21 = around 0.619048</output>

<result>13/21 = around 0.619048</result>