Rewrite y = 3x in function notation.

Function notation replaces the 'y' with a letter, usually 'f' and then puts the variable in parenthesis.

f(x) = 3x

Other equivalent examples would be:
g(x) = 3x
f(z) = 3z
h(a) = 3a

All of these represent the same function, just written in different forms of function notation. The variable can be represented as any letter and the function name can be different letters as well.

The first option would be correct if it were one half and not one