Answer:
r=5 units
Step-by-step explanation:
we have
[tex]x^{2}+y^{2} -4y=21[/tex]
Convert the equation of the circle in standard form
Complete the square twice. Remember to balance the equation by adding the same constants to each side
[tex]x^{2}+(y^{2} -4y+4)=21+4[/tex]
[tex]x^{2}+(y^{2} -4y+4)=25[/tex]
Rewrite as perfect squares
[tex]x^{2}+(y-2)^{2}=25[/tex]
center (0,2)
radius r=5 units
