The graph passes through the point (1,4.75)

Alex drew a circle with right triangle prq inscribed in it, as shown below: the figure shows a circle with points p, q, and r on it forming an inscribed triangle. side pq is a chord through the center and angle r is a right angle. arc qr measures 80 degrees.

Which geometric principle is used to justify the construction below?

Can someone me with my math problem pls my teacher is on my back about this