Explanation
In triangle ABC,
AB=AC implies that ABC is an isosceles triangle
Let one of its equal angle be x then ,
x +x+∠A = 180° (Angle Sum Property)
⇒ 2x + ∠A = 180° ...... EQ.1
Also ,∠A+x = 105° (exterior angle property)
Multiplying the above equation by 2 we get ,
2∠A+2x = 210° ......... EQ. 2
Subtracting EQ. 2 from EQ.1 ,
2∠A + 2x -2x -∠A= 210°-180° ⇒ ∠A = 30°