panjang busur AC adalah...

∠AOB = 360° - (120° +150°)
=360° - 270°
=90°

p. b. AC/p.b AB = ∠AOC/∠AOB
= ∠AOC/∠AOB × p.b AB
=120°/90° × 60 cm
= 80 cm (b)

semoga membantu