Stating that a function is 'linear' is different than stating that its graph is a line. (a)

Question:

Stating that a function is 'linear' is different than stating that its graph is a line.
(a) The function f1: R †’ R given by f1(x) = 2x - 1 has a graph that is a line. Show that it is not a linear function.
(b) The function f2: R2 †’ R given by