9514 1404 393
Answer:
∠A = 66°
Explanation:
The other base angle (∠B) of this isosceles triangle is also 57°. Of course, the total of all angles is 180°, so we have ...
∠A +∠B +∠C = 180°
∠A + 57° + 57° = 180° . . . fill in the known numbers
∠A = 66° . . . . . . . . . . . . . subtract 114° from both sides