Igot another problem on substitution on linear i'm very confused. how do i know which one to start? how do i know what to do? is there any simple explanations?
x -2y =5
3x - 5y =8

can you me do this in the simplest way ? i'm kinda struggling to get this concept : (

Don't worry, you just have to get used to these problems! You can do it girl!!

So, it doesn't matter where you start. I would start finding the x in the first equation, like this:

x - 2y = 5
x = 5 - 2y

Which is really easy, isn't it?
Now you know that x is equal to 5 - 2y, so you can substitute the x in the second equation. (You could have done the same solving by y, it doesn't matter :D )

3 ( 5 - 2y ) - 5y = 8

15 - 6y - 5y = 8

- 6y - 5y = 8 - 15

-11y = -7

y = -7/-11 = 7/11

Now you found your y! Let's substitute it in the first equation. (The beginner one, not the one that you worked on) 

x - 2 (7/11) = 5

x - 14/11 = 5

x = 5 + 14/11

x = 69/11

I know you got it now! ;) Otherwise ask me whatever you want!