Given that x=3 is the solution of the equation ax+3=7-3a, find the value of a.
已知x=3是方程ax+3=7-3a的解，求a的值．
<gadget id="calculator">7 - 3</gadget>
<output>4</output>

<gadget id="calculator">3 + 3</gadget>
<output>6</output>

<gadget id="calculator">4 / 6</gadget>
<output>2/3 = around 0.666667</output>

<result>2/3 = around 0.666667</result>