The pair of triangle shown in the diagram are similar. Similar triangle have proportional corresponding sides. So you can write a proportion connecting all given known and unknown sides:
[tex]\dfrac{23}{40}=\dfrac{x}{17}.[/tex]
From here you have that
[tex]40\cdot x=23\cdot 17,\\ \\x=\dfrac{23\cdot 17}{40}=\dfrac{391}{40}=9.775\approx 9.8\ cm.[/tex]
Answer: 9.8 cm.
