Since the triangle DEF is an equilateral triangle and x is the height of this triangle, we can use the following formula for the height of an equilateral triangle:
![h=\frac{l\sqrt[]{3}}{2}](https://tex.z-dn.net/?f=h%3D%5Cfrac%7Bl%5Csqrt%5B%5D%7B3%7D%7D%7B2%7D)
Where 'l' is the length of the triangle side.
So, using l = 13, we have:
![\begin{gathered} x=\frac{13\cdot\sqrt[]{3}}{2} \\ x=\frac{13\cdot1.73}{2} \\ x=11.2 \end{gathered}](https://tex.z-dn.net/?f=%5Cbegin%7Bgathered%7D%20x%3D%5Cfrac%7B13%5Ccdot%5Csqrt%5B%5D%7B3%7D%7D%7B2%7D%20%5C%5C%20x%3D%5Cfrac%7B13%5Ccdot1.73%7D%7B2%7D%20%5C%5C%20x%3D11.2%20%5Cend%7Bgathered%7D)
So the value of x is 11.2
