Answer:
B = 102°
Explanation:
Given isosceles triangle ABC with AB=BC, and angle A=39°, you want to know angle B.
Angle sum
The measures of angles C and A will be the same in isosceles triangle ABC with congruent sides AB and BC. The angle sum theorem tells you ...
A +B +C = 180°
A +B +A = 180° . . . . . . substitute A for congruent angle C
2A +B = 180° . . . . . . . collect terms
B = 180° -2A . . . . . . . solve for B
B = 180° -2(39°) . . . use the given angle value
B = 102° . . . . . . . . . do the arithmetic
The measure of angle B is 102°.