Are you solving for y? If so thats what im going to do for you.
4 = 8x + 5y
You want to put y by its self do your going to have to subtract 8x.
4 = 8x + 5y
-8x -8x
Then you would get this:
-8x + 4 = 5y
Now all you have to do is divide 5 from y.
-8x + 4 = 5y
5 5
Then you would get;
-8 + 4 = y
5 5
Im sorry if this didnt help but I hope it did! Have a great day! ^^