One of the lengths(which one?) is c/2 (why?), now can you find the third length, remember it is right triangle...
Without using trigonometry, express the length of the legs of a 30-60-90 triangle
in terms of the length of the hypotenuse
Draw an equilateral triangle with side and one altitude.
Code:* /|\ / | \ c / | \ / | \ / | \ * - - + - - * c/2
Can you find the third side?