A travels 4 kilometers per hour and B travels 3 kilometers per hour. When A started, B had already traveled 9 kilometers. After A chases B for 3 hours, he chases B at a speed of 5 kilometers per hour. How many hours later can A catch up with B?
甲每小时行4千米，乙每小时行3千米．甲动身时，乙已经走出了9千米．甲追乙3小时后，改以每小时5千米的速度追乙．再经过几小时甲能追上乙？
<gadget id="calculator">3 * 3</gadget>
<output>9</output>

<gadget id="calculator">4 * 3</gadget>
<output>12</output>

<gadget id="calculator">9 - 12</gadget>
<output>-3</output>

<gadget id="calculator">9 + (-3)</gadget>
<output>6</output>

<gadget id="calculator">5 - 3</gadget>
<output>2</output>

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

<result>3</result>