A(-3, -2) and B(9, 14) are the two ends of a diameter of a circle. What is the radius of the circle?

$AB=\sqrt{(9-(-3){)}^2+(14-(-2){)}^2}=\sqrt{12^2+16^2}=\sqrt{144+256}=\sqrt{400}=20$. That length is the diameter, so the radius is $20÷2=10$. Answering 20 gives the diameter instead of the radius; $4\sqrt{7}=\sqrt{112}$ comes from subtracting the squares (256 - 144) instead of adding them; and 5 comes from halving twice.