The first thing to notice is that the smaller triangle on the left is equilateral, because of the same single tickmarks on each of the three sides. For any equilateral triangle, each interior angle is 60 degrees as you have marked it on the drawing.
Once you determine that, you would solve for the 120 degree angle (60+120 = 180). Afterward, solve the equation 120+x+x = 180 to find that x = 30. The equation 120+x+x = 180 refers to adding up the three angles of the smaller triangle on the right side (with angles 120, x, x). The two missing angles are both x due to the fact that this triangle is isosceles.
Since ∆ABC is equilateral, all of its angles would be the same (60°). Since a line is 180°, the other part of ∠C is 120°. Since we also know that △BCD is isosceles, we know that angles ∠B and ∠D are equal. If a triangle is 180°, and we already know that ∠C is 120°, we can do 180-120, which is 60°. If angles ∠B and ∠D are equal, we can do 60/2=30. Therefore, ∠D is 30°.
