Explanation:
the sum of all angles around a single point on one side of a line is 180°. if they are just 2 angles (as in our case), they are called supplementary angles.
the angles ADC and ADE are such supplementary angles (around point D).
180 = 74 + angle ADC
angle ADC = 180 - 74 = 106°
angles at A and B are the same (as congruent angles just means they are equally large).
the sum of all angles in any quadrilateral (4-sided) shape is 360°.
so, we know already 3 angles, all 3 being 106°.
360 = 106 + 106 + 106 + angle C = 318 + angle C
angle C = 360 - 318 = 42°