If the triangles are similar, that means their side to side ratios will be the same.
For example, we are given the side lengths 6 and 9 to corresponding sides.
Meaning that the side to side ratios are 6:9, or 2:3.
Using this, we can find the missing side length value.
58:x = 2:3
[tex]\frac{58}{x}=\frac{2}{3}[/tex]
[tex]2x=174[/tex]
[tex]x=87[/tex]
The missing side length is 87.
Hope this helps.
頑張って!