That is a question about triangles.
In my answer I will show 2 ways to solve that question, ok? Let's go.
<h3>First way - Pythagoras's theorem</h3>
The line segment <em>x</em> divide the biggest triangle in 2 equals right triangles.
Let's choose one of them and note that is a triangule with hypotenuse equals to 8 and one cathetus equals to half of 8.
The Pythagoras's theorem says that:

<em>a </em>is the hypotenuse and <em>b</em> and <em>c </em>are cathetus.
So, in our case, we know the hypotenuse and one cathetus, let's substitute that in the expression:

Therefore, the value of x is
.
<h3>Second way - The equilateral triangle height</h3>
That way to solve the question is a consequence of the previous way.
That triangle has all the sides equals, so it is a equilateral triangle.The line segment <em>x </em>is the height of that triangle. And we can find the equilateral triangle height using that expression:

<em>h </em>is the height and <em>S </em>is the triangle's side.
So, we know that the side of our triangle is 8. Let's change S value in the expression:

Thus, the value of <em>x </em>is
.
Note that in the 2 ways we find the same result, so that answer is correct.
I hope I've helped. :D
Enjoy your studies! \o/