Answer:
D
Step-by-step explanation:
[tex]f(x) = 3x + 1[/tex]
Changing x to y and y to x:
[tex]f(x) = 3x + 1 \\\ y = 3x + 1 \\\ x = 3y - 1 \\\ 3y = x + 1 \\\ y = \frac{x + 1}{3} \\\\\ f(x) = \frac{x + 1}{3} \\\\\ f(x) = \frac{1}{3}x + \frac{1}{3}[/tex]
I hope I helped you
