Answer is b= 5
Step by step
We know the exterior angle theorem states the sum of the opposite interior angles equals the exterior angle so our equation is:
6b + 12b - 30 = 12b
Combine like terms
18b -30 = 12b
Subtract 18b from both sides to combine variables
18b - 18b -30 = 12b - 18b
Simplify
-30 = -6b
Divide both sides by -6
-30/-6 = -6/-6 b
5 = b
Check your work, substitute 5 for b
6(5) + 12(5) - 30 = 12(5)
30 + 60 -30 = 60
60 = 60
They equal so the equation is true