Explanation:
as the baseline of the abd triangle is a diameter of the circle (it goes through the center of the circle), c is the supplementary angle to 76° (it adds up to 180°), as c + 76 represents a half-circle.
c = 180 - 76 = 104°
the circle theorem says : the angle at the center is twice the angle at the circumference.
therefore a = 76/2 = 38°
similarly,
b = 104/2 = 52°
and remembering that the sum of all angles in a triangle is always 180°
d = 180 - 38 - 52 = 90°
which is true also, because all inscribed triangles in a half- circle with the baseline being the diameter, must be right-angled.