Dijawab teteh,akang, uwa, bibi, mamang sadayana:)

180° = (39 + 3x + (4x-6°))
180-39 = 7x - 6
141 = 7x - 6
147 = 7x
21 = x

∠B = 4x - 6
∠B = (4 x 21) - 6
∠B = 84 - 6
∠B = 78°

semoga membantu ........................