Final answer:
The set S can be locally described by the graph of a C^1 function in the neighborhood of (0,0,0) using the Implicit Function Theorem.
Step-by-step explanation:
The Implicit Function Theorem states that if a function F: R^3 -> R^2 is continuously differentiable and F(a,b,c) = (0,0), where (a,b,c) is some point in R^3, then in a neighborhood of (a,b,c), the set S = {(x,y,z) in R^3: F(x,y,z) = (0,0)} can be locally described by the graph of C^1 functions.
In this case, we are given F(x,y,z) = (2x+1)^2 + y - 1, (xy + y).
We can use the Implicit Function Theorem to conclude that a suitable C^1 function that describes the set S in the neighborhood of (0,0,0) has the form y = 4x(2x^2-1).