First thing to do is to notice that the two triangles are similar. To do this, you could argue that their angles are pairwise equal.

Then you need to apply what you know about similar triangles, namely that the ratio between a pair of corresponding sides is always the same, no matter which pair you consider. In particular, you know that the ratio between the hypotenuse of the big triangle and the hypotenuse of the small triangle is 2.

Hope this helps. Otherwise, feel free to ask again.