Let us consider problem number 21 in the following link

<http://www.naec.ge/images/doc/EXAMS/math_2013_ver_1_web.pdf>

It is from georgian national exam, it is written (ამოცანა 21), where word "ამოცანა" means amocana or problem. We should find angle $\angle ADE$. I have calculated angle $B$, which is equal to $87^\circ$, but is there any sign of similarity between these two triangle or how can I find it? I think I could calculate angle using arc formula, but I don't remember exactly how it is, even how can I connect arc's angle and $\angle ADE$ angle together? Please help me.