finding the y coordinate at a particular x coordinate

Is there some way to determine the corresponding y coordinate of a shape
(the edge of a circle for example)
given an x coordinate of that shape?
I know that for a circle there would be to 2 coordinates for each x
coordinate, but I'm not actually plotting a whole circle, only a quarter of
a circle.
Which brings me to me 2nd quest of how you plot a quarter of a circle with
radius of length of 1 onto an a canvas.

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