I assume 12, 13 and 15.9 are the sides of the triangle. If that is the case, then the angles are found using the Law of Cosines:
Acute scalene triangle.
Sides: a = 12 b = 13 c = 15.9
Area: T = 76.535
Perimeter: p = 40.9
Semiperimeter: s = 20.45
Angle ∠ A = α = 47.777° = 47°46'39″ = 0.834 rad
Angle ∠ B = β = 53.346° = 53°20'44″ = 0.931 rad
Angle ∠ C = γ = 78.877° = 78°52'37″ = 1.377 rad
