ANSWER
4√15
EXPLANATION
Let's keep the names of the sides shown in the picture,
The Pythagorean theorem is,
[tex]C^2=A^2+B^2[/tex]
The information we have is that A = 17 and C = 23. We have to find B,
[tex]B=\sqrt[]{C^2-A^2}[/tex]
Replace with the values,
[tex]B=\sqrt[]{23^2-17^2}=\sqrt[]{529-289}=\sqrt[]{240}=4\sqrt[]{15}[/tex]
