(a) If $R$ is a relation on set of natural numbers $\mathbb{N}$ such that $R = \{(x, y) : x, y \in \mathbb{N}, x + 2y = 25\}$, then find the Domain and Range of the relation $R$. Also, determine if it is an equivalence relation. OR (b) Determine whether the function $f : \mathbb{R} \to \mathbb{R}$ defined by $f(x) = 2x^3 - 7$, $\forall x \in \mathbb{R}$ (Real numbers) is bijective or not.