I’m not sure what happened while you were writing this, but if I’m reading it right, “My” has 18 books. If “My” has 3 times as many as 6, that also means 3 times 6, which is 18.
We know that According to Law of Sines :

where A - B - C are the Angles of the Three Vertices of the Triangle
a - b - c are the lengths of the Opposite sides of the Angles of the Respective Vertices A - B - C
From the Given Triangle we Can Notice that :



SinC = 0.682

C = 43°