We know the circle has its center at the origin, 0,0, and the point -4,-2 is on the circle, is just the distance from the center to a point on it, thus
In order to solve this you first have to calculate the radius, which is the distance from the center to any point in the circumference, to calculate this we do a trangle rectangle:
c^2= a^2+b^2
c^2=(0-(-4))^2+(0-(-2)^2
c^2=20
c=
So the diameter is two ratios put togheter, so it would be 2r=