Step-by-step explanation:
You didn't provide any context so I can't answer this. BUT!!
If you're looking at two triangles, look for the side corresponding to the unknown side. If it's an equilateral (all sides same length) triangle no need to worry. It's probably a scalene, all different lengths, because math is hard. So you'll have 3 different lengthy sides, and you can probably see which ones correspond with which based on length and position. You can use angles to help you.
Now use the sides you have the value of to determine scale factor.
Divide the length of one of the sides on one triangle by the corresponding side on the other. That's your scale factor. Now multiply it by the side that corresponds to the unknown side, and that is supposed to be your length.
Sorry couldn't help any more
