Answer: b = 69
The slashes mean the opposite angles are the same. This is and isosceles triangle, which mean two sides are the same and one is not.
A triangle = 180
We know one angle is 42 and the other two angles are the same.
Subtract 42 from 180
180 - 42 = 138
Now the 138 is going to two sides but we know they are equal so we can divide 138 by 2
138 divided by 2 = 69
So b = 69
