If x=3 is the solution of equation 5x+9a=42, then a=how much.
如果x=3是方程5x+9a=42的解，那么a=多少．
<gadget id="calculator">42 - 15</gadget>
<output>27</output>

<gadget id="calculator">27 / 9</gadget>
<output>3</output>

<result>3</result>