We have two similar right angled triangles and each has the base x/2.
Using the Pythagoras theorem we can find the value of x/2 and thus the value of x. The hypotenuse is 10, perpendicular is 8 and base is x/2. So using the theorem we can write:
Explanation: As it is an isosceles triangle and you can see it is divided into two right angled triangle, therefore, we can use Pythagoras theorem here: