math

For example, $f(x) = x + 2$ is a function. If you input 3, the output is $3 + 2 = 5$. If you input -1, the output is $-1 + 2 = 1$. Each input only has one possible output. If an input could give two different outputs, it wouldn't be a function.

For $f(x) = x + 2$, you can put any real number in, so the domain is all real numbers. Since the outputs can also be any real number, the range is all real numbers.