What theorems/ info about proportional parts in triangle and parallel lines should i use to find the missing length of a triangle? how should i set up an equation? explain with details, i will be so grateful.

Hello there!

So first, yes, you find the missing length (piece plus piece equals whole part) and proportional, you just visualize ( like let's say there are two similar shapes  that are smilier : ABCD~ FGHJ  and let's say AB is 3   FG is 9 , so i would say 3/9 . And for CD is ,..let's say 8 and HJ is 16, so 8/16. So basically:

3  =  9
--      --
8       16         <--- that's how you set up proportional

i  hope you understand , and if not, feel free to message me! 

Have a beautiful day