This is the good part of Trigonometry, actually measuring triangles. It's much better than all the identities around sine and cosine and the rest, which actually have more to do with circles than triangles.
This part of trig has a short menu:
Law of Cosines. Three ways to write it, one is
Law of Sines:
Triangle angles:
and two special cases of the Law of Cosines, the Pythagorean Theorem
and the Collinear Points Theorem:
Here we choose Triangle Angles followed by Law of Sines:
A = 180 - 105 -15 = 60 degrees
Answer: 1.8 cm
When I was a student we'd be expected to get exactly and get
But that was before online homework.