Question

how do you write f(2x) as an expression?

Answer 1

if f(x) =3x-1

Answer 2

Look at f(x) =3x-1and consider what the function f does to x. It triples the value of the variable x and then subtracts 1.

The function f will do the same to the the variable 2x.
f(2x) = 3*(2x) -1
Simplify.

Then, to be sure you have the concept, consider cranking out f(-x) if f(x) = x^2 - 1.