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!
![f(x)=\frac{1}{3} -\frac{1}{21} x](/tpl/images/0927/1704/2b760.png)
Switching the positions of the x and f(x) values...
![x=\frac{1}{3} -\frac{1}{21} f(x)^{-1}\\](/tpl/images/0927/1704/bf1a6.png)
Note: That f(x)^-1 is the proper notation for inverse functions.
![x-\frac{1}{3} =-\frac{1}{21} f(x)^{-1} \\f(x)^{-1}=-21x+7](/tpl/images/0927/1704/5b91d.png)
There you go!
Hope this helps, and don't procrastinate!