Answer:
12°
Step-by-step explanation:
Let's start with the easy first.
We know that m∠C is 39° because together ∠C and the exterior angle equal 180°. And 180 - 141 = 39.
Now, we can use this to find the remaining two angles.
180° - 39° = 141°
So, this means that we can set up m∠A + m∠B = 141°.
6x + 9 + x - 8 = 141
7x + 1 = 141
7x = 140
x = 20
Finally, we can plug in for our x value and find m∠B.
m∠B = x - 8
m∠B = 20 - 8
m∠B = 12°
