Triangle ABC is an isosceles triangle. This means that base angles A and C are the same measure.
Triangle CDE is also isosceles. This means that angles C and D are the same measure.
The hatch marks shown in both triangles clearly indicate this fact of geometry.
So, if angle C is 40 degrees, C and D must be 70 degrees each because the sum of the angles in a triangle is 180 degrees.
Angle C = 70 degrees.
