Consider Δ ABC,

We have,

∠B = 70°

And, AB = AC

Therefore, Δ ABC is an isosceles triangle.

∠B = ∠C (Angle opposite to equal sides are equal)

∠B = ∠C = 70^o

And ∠A + ∠B + ∠C = 180^o (Angles of triangle)

∠A + 70^o + 70^o = 180^o

∠A = 40^o