Explanation:
law of cosine
c² = a² + b² - 2ab×cos(C)
c is the side opposite of the C angle. a and b are the other 2 sides.
AB² = 4² + 4² = 16 + 16 = 32
AB = c = sqrt(32) = 5.656854249...
since a and b are equal (isoceles triangle), so must be the angles at A and B.
the sum of all angles in a triangle is always 180°.
180 = angle A + angle B + angle C
= angle A + angle B + 120
since angle A = angle B we get
180 = 2×angle A + 120
2×angle A = 60°
angle A = angle B = 30°.