Starting from the third number in a sequence, each number is the sum of the previous two numbers. It is known that the third number is 4.2, and the sixth number is 18.4. Find the first number. (column equation solution)
一数列从第三个数起，每个数都是前两个数的和，已知第三个数是4.2，第六个数是18.4，求第一个数．(列方程解答)
<gadget id="calculator">12.6 - 18.4</gadget>
<output>-5.8</output>

<gadget id="calculator">8.4 + (-5.8)</gadget>
<output>2.6</output>

<gadget id="calculator">1 + 1</gadget>
<output>2</output>

<gadget id="calculator">2.6 / 2</gadget>
<output>1.3</output>

<result>1.3</result>