Using the Law of Consines, we can solve for all of the angles in this triangle.
When solving for "A", we would use the equation:
a^2=b^2+c^2-2bc cos(A)
Now, plug in the values you know based on the given information:
12^2=7^2+15^2-7(15)(cos[A])
Simplify the equation:
144=49+225-105(cos[A])
144=275-105(cos{A])
-131= -105(cos[A])
cos-1= -131/-105
Therefore, "A" equals about 54.4 °
Following this same process, we can also solve for "B" and "C", however, there are different equations for each one:
When solving for "B" use: b^2=a^2+c^2-2ac cos(B)
When solving for "C" use: c^2=a^2+b^2-2ab cos(C)
Reminder, all of the angles in a triangle add of to 180°
If more clarification is needed, just let me know!