Solve the equation: 9x+21=8x+24.
解方程：9x+21=8x+24．
<gadget id="calculator">24 - 21</gadget>
<output>3</output>

<gadget id="calculator">9 - 8</gadget>
<output>1</output>

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

<result>3</result>