>>12189345f is just a variable. In computers you see sqrt() for the square root instead of a radical.
You can have a function act on x, y, itself, or any other variable you'd want. The x can easily be y.
y=2x says y and 2x are the same. f(x)=2x says the function f() acts on the variable (x) by doing the operation 2x. In this case you could even say f(x)=y because y=2x.
A function is a a binary relationship between an input and output set of numbers. The relationship is described by the 2x in f(x)=2x for this case.
