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
[tex]\text{hyp}^2=opp^2+adj^2[/tex]
This means that
[tex]\begin{gathered} HK^2=HJ^2+JK^2 \\ HK^2=10^2+10^2 \\ HK^2=100+100 \\ HK^2=200 \\ HK=\sqrt[]{200} \\ HK=10\sqrt[]{2} \end{gathered}[/tex]
