Could you please explain the answers to both questions? Thanks!

For the triangle picture:

2/6 = 5/x

2x = 30

x = 15

For the other picture:

3.2/2 = a/8

The third option applies.

Other option does not apply.
Because for Option 4,
It should be 2/3.2 = b/6.4

For option 3,
it should be b/6.4 = 5/8