Answer:
We can see here that the two sides of both triangle are equal to each other, therefore both triangles are isosceles triangles.
And also the two chords are parallel to each other, which let us know that:
b° = 40°
We also proved that that triangle is an isosceles triangle so:
b° = c° = 40°
Which makes it possible to calculate d°:
b° + c° + d° = 180°
40° + 40° + d° = 180°
d° = 180° - 40° - 40°
d° = 100°
Not sure if my thinking process make sense, but I'm quite sure about the answers.