A homeowner plots her house and yard on a computer. She determines that the circle that defines the part of the front yard that gets watered by the sprinkler is (x - 9)^2 + (y + 11)^2 = 16. What is the diameter of the circular area that gets wet?
If i got the idea right you get, from your equation (of a circle in the form #(x-h)^2+(y-k)^2=r^2#), a circle of radius #r=sqrt(16)=4# correspondent to a diameter of #2r=2×4=8#. I think you may have meters as unit.