Answer:
∠ B = 60°
Step-by-step explanation:
The exterior angle of a triangle is equal to the sum of the 2 opposite interior angles.
(10x - 45)° is an exterior angle of the triangle , then
10x - 45 = 3x + 4x = 7x ( subtract 7x from both sides )
3x - 45 = 0 ( add 45 to both sides )
3x = 45 ( divide both sides by 3 )
x = 15
Then
∠ B = 4x = 4 × 15 = 60°
