The angles can you add together to equal angle d is angle a and angle b.
Based on the diagram you sent, we can see that angle d is the exterior angle of triangle ABC.
We know that an exterior angle of a triangle is equal to the sum of the two non-adjacent interior angles of the triangle.
Therefore, we can add angle a and angle b to equal angle d.
In other words, the following equation is true:
angle d = angle a + angle b
Therefore, angle d must be equal to the sum of the other three angles in the triangle.