(a) Check whether $f: \mathbb{R} - \{3\} \to \mathbb{R}$ defined as $f(x) = \frac{x - 2}{x - 3}$ is onto or not. OR (b) Check whether $f: \mathbb{Z} \times \mathbb{Z} \to \mathbb{Z} \times \mathbb{Z}$ (where $\mathbb{Z}$ is the set of integers) defined as $f(x, y) = (2y, 3x)$ is injective or not.