Question

Find part "b".

https://media.glynlyon.com/g_geo_2012/5/01.gif

Answer 1

If you look good, you'll see three similar triangles in this figure.
All of them are 30-60-90 degree triangles.
These kind of triangles are special. The sides (from shortest to longest) have proportions:
\[a:a \sqrt{3}:2a\]
You only are given the longest side of the largest triangle. That's no problem, because we know the proportions of all the sides of this thing.
Longest = 15 = 2 times it's shortest side. So now you that one as well. And so on...