If the ends of the diameter are (-5,4) and (-6,-3), then the center of the circle is the midpoint of the said diameter, which is

((-5+(-6))/2,(4+(-3))/2) = (-5.5,0.5) -------------answer.

The radius then is

r = sqrt[(-5.5 -(-5))^2 +(0.5 -4)^2)

r = sqrt[(-0.5)^2 +(-3.5)^2]

r = sqrt[0.25 +12.25]

r = sqrt(12.5) = 3.535534 ---------------answer.