We will determine if the functions represent a circle with a real non-zero radius as follows:
b)
[tex]3x^2+3y^2-11x=-91[/tex]
So, we try to factor it to determine the nature of the quadratic equation and since it cannot be factored or even plotted, we will have that it is not a circle with a real non-zero radius.
c)
[tex]4x^2+4y^2+18x-8y=-85[/tex]
So, we try to factor it:
[tex]\Rightarrow2(2x^2+9x+2y^2-4y)=-85[/tex]
We can factor in a limited fashion, but we can conclude that it is not a circle with a non-zero radius given the nature of the factorization.