If you start with
[tex]f(x)=y=5x[/tex]
In order to invert the function, you have to switch y and x, and then solve for y again. In this case, after the switch we have
[tex]x=5y[/tex]
And solving by y we have
[tex]y=\dfrac{1}{5}x[/tex]
This is your inverse function.
