You want the measure of the inscribed angle in a semicircle.
Inscribed angle
An inscribed angle has a measure equal to half the arc it subtends. Angle h intercepts the circle at the endpoints of a diameter, so subtends an arc of 180° (half a circle).
The measure of angle h is half of 180°, or 90°.
h = 90°
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Additional comment
The fact that a triangle inscribed in a semicircle is a right triangle is useful for a variety of purposes. It is also useful to note that the circle center is the circumcenter, and is the midpoint of the hypotenuse.
