Answer:
c. 19.73
Step-by-step explanation:
The Cosine rule shows states that:
a²=b²+c²-2bcCos A, where a, b and c are the sides of the triangle and A is the angle at vertex A.
A=108°
b=9
c=15
Substituting with the values above gives:
a²=9²+15²-(2×9×15 Cos 108)
a²=389.4346
a=19.73