The sum of all the angles in a triangle is 180 degrees. The triangle in the picture is also an equilateral triangle because of the markings on it (the three arches and ticks show that the sides with the markings are congruent.)
So since an equilateral triangle has 3 angles and the angles are congruent, we can divide 180 by 3 to find out how many degrees each angle is; 60 degrees.
Since all angles are equal, angle A = angle B = angle C = 60 degrees.
