Question 77711
I'm not sure this question was typed right 

Here is an equilateral triangle with a line of sequins from one of its edges to the opposite vertex.  Also, it's the midpoint of the edge from which I'm starting.  

If that is not what the question is asking, then this is not going to be the right answer.

Here is an "ascii art" diagram of an equilateral triangle with a line of sequins sewn to the opposite vertex from one of the lines.  The sequins here are ooo and I am labeling some of the lengths here.

The <pre>
   \ 
      \
         \  
 </pre> is really supposed to be a straight line but I am not very good at ascii art so please imagine it is a straight line.  You will want to draw a better diagram for yourself so you can see it better.

<pre>
   |\
   |    \     12
 6 |        \
   |            \
   |        S       \
   |oooooooooooooooooo
   |                /
 6 |            /
   |        /  12
   |    /
   |/
</pre>

I have labeled that line of sequins with the letter S to represent its length.

Ok so how long is that line of sequins?  This is an application of the Pythagorean Theorem.  (The Pythagorean Theorem is true for people who believe in God or the Flying Spaghetti Monster or Buddha or whatever.)  

{{{6^2 + S^2 = 12^2}}}
{{{36 + S^2 = 144}}}
{{{S^2 = 108}}}
{{{S = sqrt(108)}}}