Answer:
0.51508
Explanation:
Since you didn't post the picture of the graph, im assuming the circle is on the middle of the graph i.e. the center at (0,0)
A unit circle means that the the radius of the circle is 1, therefore the highest y value would be 1, and the lowest y value would be -1.
The equation of a circle is x^2 + y^2 = r^2, r being the radius. Since the radius of a unit circle is 1, the equation would be x^2 + y^2 = 1
So to find y, just plug in the x.
(6/7)^2 + y^2 = 1
1 - (6/7)^2 = y^2
sqrt(1-(6/7)^2) = 0.51508
If the center of the circle is not (0,0), the equation would be (x-h)^2 + (y-k)^2 = r^2, where (h,k) is the coordinate of the center.
For further question, don't hesitate to comment. I would appreciate it if you would notify me if the circle was indeed in the center or not.
Hope that answers it.