Question 965862
3, -6, 12, 4, 20, 

<pre><b>
Look at the red numbers below.  They go <font color="red">-9</font>,<font color="red">-8</font>,<font color="red">-7</font>,<font color="red">-6</font>,...
Look at the blue numbers below. They go <font color="blue">18</font>,<font color="blue">16</font>,<font color="blue">14</font>,<font color="blue">12</font>,...</b> 
<b>
3 + <font color="red">-9</font> = -6
         -6 + <font color="blue">18</font> = 12
                   12 + <font color="red">-8</font> = 4
                             4 + <font color="blue">16</font> = 20
                                      20 + <font color="red">-7</font> = 13
                                                13 + <font color="blue">14</font> = 27
                                                          27 + <font color="red">-6</font> = 21
                                                                    21 + <font color="blue">12</font>  

Notice also that each blue number is -2 times the preceding red number.

Edwin</pre></b>