Verified answer

tan30° = BC/AB ⇒BC = AB x tan30°   (tan30° = 0.577)

                                    = 11 x 0.577

                                    = 6.35 m

BC = 6.35 m

BT = BC + CT = 6.35 + 0.80 = 7.15 m

tan30° = BT/RB ⇒ RB = BT/tan30°

                                    = 7.15/0.577

                                    = 12.39 m

RB = RA + AB ⇒ RA = RB - AB = 12.39 - 11 = 1.39 m ≈ 1.40 m

soit  RA = 140 cm