Question

how do i set this up

Answer 1

you have a "little triangle" (the top one) that is similar to the big enclosing triangle
set up a proportion of little side / little side = big side/ big side

Answer 2

x/5=4/8

Answer 3

pretty close, but I think the big "slanted side" is 8+4 not just 8

Answer 4

i dont get your point

Answer 5

if we draw the big triangle without the little one, it looks like this: