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Cari nilai x terlebih dahulu
2x + 50° = 5x - 40°
- 3x = - 90°
x = 30°

Besar sudut B = 180° - (50° + 2x)
=180°- (50° + 2•30°) = 180 - (50° + 60°)
=180°- 110° = 70°