You're expected to know the general equation of a circle.
Given: center and radius
. . . . . the equation is: .
Find the center and radius of each circle.
Write the standard form of the equation of each circle.
 CIRCLE A PICTURE HAS:
Point (1, 0) on the circle and point (1, 2) in what appears to be the center of the circle.
You already know the center: .
The radius is the distance from the center to the point .
. . You can see that the distance is , right?
Write the equation: .
 CIRCLE B PICTURE HAS:
Point (0, 1) on the circle and point (2, 3) also on the circle.
There is not enough information to determine a specific circle.
My guess is that these two points are the ends of a diameter.
If that's true, the center is the midpoint between the two points.
. . Use the Midpoint Formula to find it.
Then use the Distance Formula to find the distance from to one of the points.
. . That, of course, is the radius.