Can someone please tell me how to solve this problem? (below) Thanks?

Question

Find the equation of the sphere with center (2,-1,-3) and satisfying the given condition:
a) Tangent to the xy-plane
b) Tangent to the xz-plane
c) Tangent to the yz-plane

Please show me the details of working it out and not just the answers. Thanks

Answer 1

a) Tangent to the xy-plane
(x - 2)^2 + (y + 1)^2 + (z + 3)^2 = 9
b) Tangent to the xz-plane
(x - 2)^2 + (y + 1)^2 + (z + 3)^2 = 1
c) Tangent to the yz-plane
(x - 2)^2 + (y + 1)^2 + (z + 3)^2 = 4