Explanation:
a kite is a symmetrical object. the left side has the exact same measurements as the right side.
the diagonals are crossing each other in 90° angles.
and as you can see, every kite is also split into these 4 right-angled triangles.
angle 1 is therefore = 39°.
and for angle 3 we remember, that the sum of all angles in a triangle is always 180°.
so,
180 = 39 + 90 + angle 3
51° = angle 3