I can't figure out what's behind it.

It took me years to figure out that I had been adopted. 
自分が養子だと知ったのは何年もたってからだった。

Even I can figure that one out. 
私にだって、それくらいのことは分かります。

I can't figure out what's behind it. 
原因がよく分からないんですが。

No one can figure him out.
誰も彼を理解できません。

I can't figure out how to do it. 
それをどうすればよいのか分からない。

Does anyone know how to solve this problem? I've been trying to figure it out since yesterday. 
誰かこの問題の解き方が分かりますか。私は昨日からずっと悩んでいます。

It took me three days to figure out the problem. 
 問題の原因［解決法］が分かるまで三日かかりました。