Explanation:
the main triangle is an isoceles triangle (both legs are equally long). that means that the height y bergen the 2 legs splits the baseline in half.
therefore,
x = 10/2 = 5
Pythagoras gives us y.
c² = a² + b²
c being the Hypotenuse (the side opposite of the 90° angle). in our case 10.
a and b are the legs. in our case x and y.
10² = 5² + y²
100 = 25 + y²
75 = y²
y = sqrt(75) = 8.660254038...