Question 828378
<pre> 

Given: b = 6, AC = 30, and AB = 2BC

Therefore AB = b-a, BC = c-b, AC = c-a  

 A                   B         C
---------------------------------
a=?                 b=6       c=?

  AB = 2BC                AC = 30
 b-a = 2(c-b)            c-a = 30
 6-a = 2(c-6)
 6-a = 2c-12
  18 = 2c+a
2c+a = 18

So we have the system of two equations in two unknowns:

{{{system(2c+a=18,c-a=30)}}}

Add the two equations and the a's cancel:

     3c = 48
      c = 16

Substitute in 

   2c+a = 18
2(16)+a = 18
   32+a = 18
      a = -14

Edwin</pre>