http://prntscr.com/l8qhmm
the answer is 2c because the base is 4c and half the base is where the vaiable a is.
Drop a plumb bob from A to show a = 2c or c= a/2
there's really no need for Pythagorean Theorem because its a equilateral triangle, so it's just 4c/2 == 2c
Thanx....I had already edited out my error....
+127 +127 
