all sides are congruent, meaning is a rhombus, so ∡2 = ∡1, thus ∡2 = 160°.
is a quadrilateral, so all interior angles add up to 360°, so, since ∡1 and ∡2 add up to 320°, that means the other two angles add up to 40°, so each one is 20° then.
now, that diagonal there, is cutting one of those angles in half, so one half of 20° is just 10°, and it so happens that ∡3 is one of those halves.