The floor function $f(x) = [x]$ maps every real number to the greatest integer less than or equal to $x$.
Different real numbers can have the same floor value, so it is not one-to-one,
but for every integer $n \in \mathbb{Z}$, there exists an $x \in \mathbb{R}$ such that $[x]=n$.
Hence, it is onto but not one-to-one.
For $f(x)$ to be real, both radicals must be defined:
$x - 3 \ge 0 \quad \text{and} \quad x - 4 \ge 0$
$\Rightarrow x \ge 4$
So the domain is $[4, \infty)$.
$f(x) = \dfrac{1}{x}$ is not defined for $x = 0$, hence it’s not a function from $\mathbb{R} \to \mathbb{R}$.
If domain were $\mathbb{R} - {0}$, then it would be bijective.
Two men on a 3-D surface want to meet each other. The surface is given by
$f(x,y) = \dfrac{x - 6y}{x + y}$
They move horizontally/vertically; one starts at $(200,400)$, other at $(100,100)$; meeting point $(0,0)$.