It's important to know that the given triangle ABC is isosceles because it has two equal sides AC = BC. Isosceles triangles have equal base angles, which means angles A and B are equal.
Using the interior angles theorem, we have
[tex]\begin{gathered} m\angle A+m\angle B+m\angle C=180 \\ m\angle A+m\angle A+40=180 \\ 2\cdot m\angle A=180-40 \\ m\angle A=\frac{140}{2} \\ m\angle A=70 \end{gathered}[/tex]
Therefore, angle A measures 70°.
