The Cookie Monster encounters a cookie whose boundary is the equation x^2+y^2 + 21 = 4x + 18 y and is very confused. He wants to know if this cookie is a lunch-sized cookie or a snack-sized cookie. What is the radius of this cookie?
Put it into standard form,
you do this by completing the square for the y terms.
Then you isolate the constant, then square root it to find the radius.