Let $f : \mathbb{R} \to \mathbb{R}$ be defined by $f(x) = 4x^{2}+2$ and set $y = f(x)$.

(a) The level sets of $f$ are subsets of $\mathbb{R}^k$ for $k =$

(b) Find the level set of $f$ at level $y = 6$. If there is more than one answer, enter your answers as a comma separated list.