I have attached a screenshot of the question along with the available choices.
Answer:
∠A = 78°
Step-by-step explanation:
We are given that ΔABC is isosceles-angled triangle.
This means that:
AB = AC
∠ABC = ∠ACB
We are given that ∠ABC = 51°
This means that ∠ACB = 51°
Now, the sum of measures of the angles of a triangle = 180°
This means that:
∠A + ∠ABC + ∠ACB = 180
∠A + 51 + 51 = 180
∠A + 102 = 180
∠A = 180 - 102
∠A = 78°
Hope this helps :)