Question 572214
Here is the needed equation:

(x/2)^2 + (21)^2 = x^2

After solving for x, I got x is approximately the square root of 588 as the value of one side of your equilateral triangle.


Of course, sqrt{588} can be reduced to 14(sqrt{3}). 


So, the length of the side of your triangle is 14(sqrt{3}).