The diameter is a vertical line (x=3) and is of length 9 (= 5-(-4)). The radius is half the diameter (so is 4.5), and the center is 4.5 units above the bottom end of the diameter, so is at (3, 0.5). The square of the radius will be 4.5² = 20.25.
A circle with center (h, k) and radius r has formula
... (x -h)² +(y -k)² = r²
The equation of this circle can be written as
... (x -3)² +(y -0.5)² = 20.25