Use this following formula to solve the problem
a² + b² - (2ab) cos C = c²
From the question, we can determine
a = 9
b = 12
c = 15
and the question is the angle C (angle between a and b)
Input the numbers between the formula
a² + b² - (2ab) cos C = c²
9² + 12² - (2 × 9 × 12) cos C = 15²
81 + 144 - 216 cos C = 225
225 - 216 cos C = 225
-216 cos C = 0
cos C = 0
∠C = 90°
Angle between the 2 shortest sides is 90°
