Which of the following functions from $Z$ into $Z$ are bijections?

Here $f(x) = x + 2 \Rightarrow f\left( {{x_1}} \right) = f\left( {{x_2}} \right)$

${x_1} + 2 = {x_2} + 2 \Rightarrow {x_1} = {x_2}$
Let $y = x + 2$

$x = y - 2 \in Z,\forall y \in x$

Hence we can say that, $f(x)$ is one-one and onto.