Explanation:
remember the law of cosine :
c² = a² + b² - 2ab×cos(angle)
with c being the side opposite of the angle.
a and b are the other 2 sides.
so, in our case
x² = 12² + 15² - 2×12×15×cos(144) =
= 144 + 225 - 360×cos(144) =
= 369 - 360×-0.809016994... =
= 369 + 291.246118... = 660.246118...
x = 25.69525478... ≈ 25.7