Answer:
you need to use Pythagoras therum.
to help you figure it out, you can draw a 90° angled triangle.
6,8,10 Is correct because:

x= 10 is correct
9,12,15 is also correct. try to do what I did with 6,8,10...
16,30,34 is also correct
the wrong answer is 10,20,30
If you need anymore help tell me.