SOLUTION
From the figure below,
HJK is an isoceles triangles with sides HJ = JK.
Also the triangle HJK is also a right angle triangle with angle 90 degrees at J.
This means that side HK is the hypotenuse.
Since HJ = JK, and HJ = 10, then JK = 10 also.
We will use the Pythagorean theorem to find HK.
From Pythagorean theorem
This means that