Answer:
Gah! Inverses!
Step-by-step explanation:
This is a pretty simple concept, just switch the x and f(x) values!
If you don't understand, look at how I solve it!
[tex]f(x)=\frac{1}{3} -\frac{1}{21} x[/tex]
Switching the positions of the x and f(x) values...
[tex]x=\frac{1}{3} -\frac{1}{21} f(x)^{-1}\\[/tex]
Note: That f(x)^-1 is the proper notation for inverse functions.
[tex]x-\frac{1}{3} =-\frac{1}{21} f(x)^{-1} \\f(x)^{-1}=-21x+7[/tex]
There you go!
Hope this helps, and don't procrastinate!
