D
using the Cosine rule in ΔABC
let a = BC, b = AC and c = AB, then
b² = a² + c² - (2ac cosB )
substitute appropriate values into the formula
b² = 7² + 9² - (2 × 7 × 9 × cos 73° )
= 49 + 81 - 126 cos 73°
= 130 - 36.84 = 93.16
⇒ b = [tex]\sqrt{93.16}[/tex] ≈ 9.7 → D
