Answer:
132°
Step-by-step explanation:
Quadrilateral ABCD is inscribed in a circle,
So, it is a cyclic quadrilateral.
Opposite angles of a cyclic quadrilateral are supplementary.
[tex] \implies \: x \degree + (3x - 12) \degree = 180 \degree \\ \\ \implies \: (4x - 12) \degree = 180 \degree \\ \\ \implies \: 4x - 12 = 180 \\ \\ \implies \: 4x = 180 + 12 \\ \\ \implies \: 4x = 192 \\ \\ \implies \: x = \frac{192}{4} \\ \\ \implies \: x = 48\\\\[/tex]
(3x-12)° = (3*48 - 12)° = (144 - 12)° = 132°
[tex]\implies \: m\angle B = 132\degree\\\\[/tex]
