Answer:
B=30°
Step-by-step explanation:
Recall that the sum of interior angles of a triangle equals to 180°. So basically
[tex]A+B+\alpha+\beta=180^{\circ}[/tex]
[tex] \implies B + {70}^{ \circ} + {80}^{ \circ} = {180}^{ \circ} \\ \implies B = {180}^{ \circ} - {150}^{ \circ} \\ \implies B = \boxed{{30}^{ \circ} }[/tex]
